Did you go to the seat reservations page on interrail.eu (website only) ? I'm able to book reservations for August : 30€ +2€ booking fee

If there are more than 2 people you could also book here (4€ booking fee per order) : https://www.b-europe.com/EN/Booking/Pass#TravelWish

Thank you @thibcabe - yes I was on the reservation page of the Interrail website.  I can see the timetable on the left but when I select the journey it doesn’t do anything - the window on the right remains the same.  

Did you add your last name and pass number in order to create a trip ? (no need to activate the pass to do that)

Thank you @thibcabe - I’ve just managed to book the reservations!   I hadn’t noticed that you had to tick the tiny box first before it would add it to the cart.  It all feels like quite a minefield to get your head around but hopefully it will get easier!    I was hoping to leave our return trip to the UK in late August a bit open for flexibility but do you think Eurostar tends to get booked up at that time of year?  

Eurostar is a true bottleneck so I would advise to book it a few weeks in advance. Don't expect available seats a few days prior (unfortunately). There are often more seats from Brussels and Lille than Paris

Otherwise you could take a ferry from Calais (30£ afaik), there is always more availability last-minute

If you're ending your trip in or around the Netherlands you could also consider a daytime or overnight ferry from Hoek van Holland (Rotterdam) to Harwich (30% discount with Interrail). I'm actually taking that route this summer : Eurostar to London and back to Switzerland using the overnight ferry to the Netherlands ;-)

Edit : yes things will definitely be easier as soon as you've passed Eurostar. Which countries are you travelling to ?

France and Spain are a bit painful but otherwise it's fantastic. The first one has mandatory reservations on TGVs (limited 10€, then 20€) and the latter has reservations that can mainly be bought only while in Spain.

On the other hand Belgium, the Netherlands, Germany, Switzerland, Austria are perfect. Lots of trains and simply board any one you want. Italy also has 10€ mandatory reservations but there is often availability on the day + you can always rely on reservation-free regional trains

@thibcabe thank you so much for taking the time to reply, that is all so useful to know.   We are planning London-Paris-overnight Milan-la Spezia-Florence-Venice-overnight Paris-London.  So we are mainly in Italy.    Would you book all the seat reservations before we leave the UK or do it on the spot?  I guess for the overnight trains there may be less availability for certain times so perhaps we need to book those at least in advance.   I can’t quite decide about the overnight trains.   I like the idea of saving money and time but I wonder whether we will arrive with little sleep and less able to enjoy the following day.

Unfortunately there are no overnight trains between France and Italy for a few years so here are the options :

- Paris - Nice, from there Milan is 5h10 away or La Spezia is 5h20 away the next day. Quite an older train so don't know about comfort, maybe someone can help ?

If you don't like this option, take an afternoon TGV to Marseille or Nice and spend the night there OR a TGV to Strasbourg/Mulhouse and then overnight somewhere in Switzerland. There are great connections between Switzerland and Milan. Let me know if you need advice.

- Venice - Stuttgart, comfy ÖBB Nightjet. I'd recommend a 2-people private sleeper (if you're 2 obviously). It costs about 70€ per person but it saves a night accomodation. This one can easily be booked on nightjet.com (add Interrail discount).

The next day Stuttgart - Cologne/Frankfurt - Brussels and Eurostar from Brussels. In my opinion way better than going through Paris again. You could arrive in London at 16:00 or 17:00 without problem

Night trains sell out weeks in advance, there's less capacity so yes it's best to book quite in advance. I'd say as soon as your travel dates are settled.

For other trains within Italy there will be availability last-minute. Maybe not for the train you'd like but for the next one 1 hour later. There are also always reservation-free regional trains on the same route so Italy is great in that regard. I'd say if you want to take a specific train, book it 2-3 days in advance.

Within Italy use tickets.oebb.at : add Interrail as a discount and select one-way tickets. It's the easiest way to book those trains + there's no booking fee