It's not as straightforward as it should but Interrail is still a great product. Buying regular tickets between multiple companies isn't easier I'd say.

Have a look at these helpful guides :

https://www.seat61.com/how-to-use-an-interrail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

Interrail/Eurail websites shouldn't be used to book reservations. The overview is confusing, there are extra fees and sometimes they only offer paper reservations sent by mail while they're easily available online elsewhere.

Rail Planner is rarely updated and sometimes wrong, for example on the "mandatory" reservations in the UK.

Have you read the answers in your other post? To leave the UK you don't need anything other than your Eurostar reservation (and a pass of course).

If you want real helpful advice, mention route and travel date. We'll all gladly help.

It’s not always as simple as it should be, but it’s worth sticking with it. There’s a good summary of reservation requirements and costs here:

https://www.seat61.com/interrail-and-eurail-reservations.htm

The Eurostar, for example, can be booked on Rail Europe website (not mobile) by adding that you have a pass in the search box. It should then be €38 for a first class passholder fare to Paris or Brussels, or €43 to the Netherlands. No reservation needed for UK domestic travel.

If you post your route, somebody will be able to provide more specific advice.

@mikeb If you let the Community know what you need help with, you will get advice. 

Its this disconnect between the booking and the seat reservation. Booked the journey and then, when trying to book the mandatory Eurostar seat via Eurostar, it was listed as £60 for a seat. Even Chester/Euston, the mandatory seat reservation could only be booked at Virgin, which proved impossible. It is the least user friendly system I have ever come across in three decades as a travel agent/tour operator. I have rebooked my whole itinerary with air out/back and individual rail jouneys via the websites of national carriers, outwith the pass - all in under a day. I am now finding it impossible to even register for a refund by following the instructions on the site. Anyone got a postal address I can use for snailmail ?

By "Booked”, you mean you registered the journey in the app? That doesn't book anything, that's the digital equivalent of writing a train on the paper trip report and isn’t needed to book a reservation.

That can be booked on www.raileurope.com (desktop website only). That's cheaper and also more reliable than via Interrail. If that doesn't work for you, then please mention date and departure time.

That reservation is optional, but can be booked via GWR, see this explanation:

https://www.seat61.com/interrail-and-eurail-reservations.htm#UK

Again, if that doesn't work for you, then please mention date and departure time.

You can thank the operators for that. They generally don't make it easy to book pass reservations (with a few exceptions). It's one of the reasons the page about pass reservations on Seat61 exists, and we provided you a link to that page 10 days ago. The information I wrote above is on that page.

To get a refund, log in on on your Interrail account, go to the Order overview > Pass details > Refunds and exchanges. There, you can request a refund.