I would be glad of some advice please on the above subject thank you.

Hello David,

This is quite an ambitious journey you are planning but here goes.

The best way to look at your itinerary and how many travel days etc is firstly to consider your own stamina and preferences on travelling. 

There are relatively few routings for your plan and you can only use 2 of your travel days in the UK.

The most likely routing would be Swansea - London - Lille or Paris - Marseilles - Nice - Ventimiglia - Rome - Brindisi. 

I would suggest you take 4 travel days each way, this would be:

Day 1 Swansea to Lille

Day 2 Lille to Marseilles direct TGV

Day 3 Marseilles - Nice - Ventimiglia - Rome (via Genoa

Day 4 Rome to Brindisi.

Return could be same routing or Brindisi - Verona - Munich - Brussels - Swansea, again taking 4 days.

These will be reasonably long travel days but all on modern high speed and Inter City trains so reasonably stress free.
Technically you could do it in 3 travel days each way but that would be very long and needs late arrivals and early starts. 
Obviously you can stay longer in each of the stops (Lille/Marseilles or Nice and Rome) so a 12 to 14 night holiday would sound about right.

You might consider finding a kindred spirit to do it with you.

You would need to add about 100 euro each way to the pass cost to cover reservations.

I would also suggest First Class for comfort and space. The oncost is relatively small for the gain. 

I would also consider a continuous 15 day pass for about 40 euro more than the 10 day flexi pass so you really can enjoy the rail experience on even local trains around Brindisi without needing to count travel days. (Think of it like having a bus pass). You could then even do some of the legs over a couple of days using reservation free regional trains.

Another option (which I have done a couple of times) is to fly out and meander back with a 5 day pass. I did flight to Catania then bus to Siracusa - 2 nights. IC train to Salerno - 1 night, Frecciarossa to Bologna - 2 nights, Eurocity  to Nuremberg - 3 nights, Nuremberg - Brussels - 1 night, Brussels to Leeds. 

Hey David

Making seat reservations in station isn't something we would recommend, especially for Eurostar. That booking you should make as soon as you can.

For the rest of the trains, and given the relatively calm travel period that time of the year, you can risk buying reservations at the ticket office. But there's always a risk of a fully booked train. For the long distance stretches, I would recommend to make your reservation in advance as well. Especially with SNCF's TGVs

Best do your planning in bits and pieces. Don't try to find a route from Swansea to Brindisi. Split it up, it's a complex journey, like @Yorkie suggested. I a'so agree you should expand the number of your travel days. It will give you more flexibility and you can e.g. go via France and come back via Switzerland, Germany and Belgium. It takes longer time, but these are much friendlier interrail countries, without mandatory reservations and still many ticket offices!

Best use the local train planners to plan your journey. SNCF in France, Trenitalia in Italy. Deutsche Bahn has a really good rail planner, encompassing most of Europe. Don't use the rail planner app of interrail. It doesn't update enough to be reliable and it has some mistakes. 

There's a good guide on how to get reservations, made by community members. Please read it attentively. 

Once you decided which itinerary suits you, we'll gladly help you here to figure all the reservations and timetable stuff out with you. :) 

Thank you for your kind replies. I will speak to you tomorrow. Good night. David.
David Shaw.

https://www.seat61.com/

Hi David,

the link is a great source of info on train travelling.

I'd second Yorkie above in getting a 15 day continuous pass (if it is possible to exchange for the one you've bought). For some more money you will have much more hassle-free travelling, you can split up your journey in shorter sections and be more flexible and avoid worrying about what will happen if you run out of travel days.

Enjoy your trip!

Well done on planning a trip like this! You probably have a reason for going to Brindisi but Lecce a bit further South is much more interesting! Book the Eurostar asap especially if you are going near school half term. I’d also book a coach to London in case rail strikes disrupt your journey. If you book a flexible ticket on National Express you can cancel it once you’re sure the trains will be running!

Here is information about booking the Eurostar.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

David, as mentioned by Veronika1278, seat61 is excellent for help with planning rail journeys.
This page on there details how to get from London to Brindisi (and back):
https://www.seat61.com/Italy.htm#london-to-rimini-ancona-bari-by-train

One possibility might be to do one leg of your journey by plane, eg Bristol to Rome. 
I’ve just had a quick look on Google Flights and with easyjet a direct flight on Feb 7 would cost 22GBP plus bag. (Mondays & Tuesdays in February are the cheapest days to fly.)
https://www.google.com/travel/flights

If you did that, you could then get a train down to Brindisi.

..

By the way, if you flew both ways, you could even consider not using the interrail pass for this trip (and use it later in the year for a different rail trip) when there’s more daylight.
Rome to Brindisi can cost as little as 15eur (about 13GBP) each way. See Trenitalia:  https://www.trenitalia.com/en.html
 

Some food for thought: this flight combination is currently about 45GBP (plus 60-70GBP for a large bag):

Tues 7 Feb Bristol-Rome 13.55-17.30, stay a night or two in Rome (or Naples), then travel by train to Brindisi
Train back from Brindisi to Rome & fly home on Tues 14 Feb Rome-Bristol 18.15-20.05