Look at the tutorial on this site about using a global digital pass. Don’t activate it until the very last minute is the community advice. You reserve Manchester to London from any UK ticket office. The difficult bit is Eurostar. Book these seats on the SNCB international site and as soon as you can as there are very few passholder places on each train. You just book two seats you don’t need pass numbers or an activated pass. You must travel from Manchester to Paris on one day. 
If you’re travelling some time in the future and TCs allow-change to a paper pass. They’re easier for the non technical. I'm in my 60s and travel a lot and prefer the paper ones. If you get stuck with Eurostar the experts on this site will help but we need full travel plans 

Many thanks for that. I have booked the UK leg directly with the rail company and paid £8 reservation fee. I have also managed to book the Eurostar to Paris through Eurostar with a reservation fee of 80 euros. (for 2)Seems a bit steep considering we have already paid for a Global Pass and no doubt we will incur another seat reservation fee when we depart Gare de Lyon. I note that when we reach Switzerland no fees are applicable on their rail network and the pass includes travel on the Golden Pass Line which is a nice surprise. Once again, I appreciate your assistance.

Regards.

Eurostar do not sell passholder reservations on their website.

Who charged you £8 for a seat reservation? They are free from all manned UK stations or online from GWR.com

ACP Rail. LNER couldn't seem to organise the route we wanted and I was getting nowhere trying to book through the Interrail site. The rail planner on the Interrail site said seat reservations were required and so I assumed there would be a cost involved.

Welcome to Interrailing from a fellow U3A (university of the Third Age) couple who have just finished our first season of interrailing after years of cruising and package holidays.

Despite what has been said, IMO (In my opinion) stick with your mobile pass, just take a bit of time to practice with it.

There are plenty of guides and advice in the community and I often try to simplify the process for new to Interrailers, so please let me know if you would like help with the App.

As for planning your journey some simple tips:

Do not overload your time on trains - we had 2 * 5 day breaks planned, both of which included a low cost flight to the start point and a Eurostar and LNER return to the UK. On train days we travelled between 4 and 9 hours, but usually put at least one city stay between them.

We compromised a bit on quality of hotel/B&B/Apartment to keep costs reasonable - between 3 and 4 star ratings. 

We tried, with reasonable success, to find accommodation near the main station so we didn’t need taxis or public transport with luggage.

As with any touring holiday do plenty of research.

Always have a plan B in case trains are late or cancelled. At our age 10 minutes to change trains is not a good idea - we do not need the stress - There is always a refreshment point nearby if you have a wait - just people watch.

Beware of night trains - OK for backpackers and you pay far more than a hotel if you want privacy and a wash basin on most night trains, even with a pass.

Just ask if you want more help.

As a general rule trains with mandatory reservations do not have standing passengers. In the UK there has been an ongoing debate about UK trains and reservations. Technically all the main UK operators have optional, not mandatory reservations, despite the Arriva site saying they are needed for their trains from the NW. LNER will reserve all the seats on a train except coach C, so you can see the problem of anybody getting on down the line. 

However reservations for pass travellers are easy to get from their local station ticket office or online, especially easy with LNER for their trains. You can even choose your seats if you don’t like their choice.

Once on the mainland each operator has their own routes to getting reservations and there are excellent guides in the community.

Reservations are not required for any domestic day train in GB but LNER / RDG are cheating by saying in their timetable data that they are compulsory and that's why the rail planner app is showing that (and other planners too). Nevertheless, they're very much recommended on LNER and free of charge.

Interrail have been told multiple times that the information on their website about mandatory reservations on LNER and TPE is wrong (with references to the operators’ websites) but they don't feel like correcting it for some reason.

Many thanks for all the very useful information. Like any new venture it's only when experiencing for the first time that you realise you need information/advice from experienced travellers.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

Many thanks Anna.