Yes, make sure you've added in the full cost of the reservations on Eurostar, the French TGV and the Spanish high speed trains. And checked availability.
Seat 61 has a good summary of reservation fees and how to actually buy them.
https://www.seat61.com/interrail-and-eurail-reservations.htm
But yes, a pass can be cheaper than individual tickets, especially when the tickets are at the high end.
Eurostar will add a fair bit to the costs (30€ each way) but as tickets are really expensive the pass is generally good value. However as advised please check if there's still availability.
You could enter Spain one-way via San Sebastian and the other via Barcelona. The former is cheaper but doesn't show on all planners as the hop across the border isn't included :
- Euskotren Hendaye - San Sebastian every 30 min 3-4€
Don't hesitate to ask further questions mentioning train and travel date ! :)
There are some tricks too : like taking a London - Lille-Europe Eurostar followed by a TGV to Bordeaux or Montpellier to avoid the transfer faff in Paris.
For example you could do Stroud - Bordeaux on day 1 and Bordeaux - Salamanca a day or two later. :)
This is really helpful, thank you!
Ok, if it’s alright asking more:
There are two adults travelling.
Our constraints are:
- We must be in Salamanca on the morning of Monday the 23rd Oct. My partner is talking at a conference, so ideally we’d have had a good night’s sleep the night before!
- We need to be back in Stroud no later than the evening of the 30th, but can come back on the 28th or 29th (if it alters the price a lot)
- We’d like to spend a couple of days in Salamanca but then a couple of days _somewhere else_, maybe Madrid. We don’t _really_ mind where, this is just the sight seeing part of the trip, so somewhere fairly interesting.
Advice within these constrains very welcome!
I should also say that we’re very happy with sleeper trains...saves having to find a hotel as well!
There are sleeper trains from Paris to Perpignan and the Spanish border, or to Latour de Carol in the Pyrenees. But I don't think they'll be fantastically helpful for your journey, unless you want to take things slowly. You still spend most of the day before and most of the day after traveling.
Seat61 describes the options to Madrid pretty well. It's just a case of pitching a city to overnight for a couple of days. Bordeaux, Bilbao, Barcelona, Montpellier, Lyon etc.
https://www.seat61.com/Spain.htm#london-to-madrid-by-train
Thanks for your answer ! Some advice :
Unfortunately not all Spanish trains can be booked online (until recently it was none). It’s quite annoying and that’s why Spain is one of the worst countries for Interrail. RENFE does it best to prevent the use of passes... 
More info about seat reservations can be found here : https://www.seat61.com/interrail-and-eurail-reservations.htm
Mid-morning Eurostar trains (the most convenient ones) are already sold out on Saturday 21st October. It only reduces a bit your options but the trip is possible, don’t worry !
High-speed trains will also be busy in France that weekend for the start of the autumn holidays.
Those were the bad news.
Since high-speed train tickets are quite expensive you’ll save a lot of money with the pass !
Now I’ll add 2 possible itineraries. There are more : you could for example leave earlier on Friday and spend the night in Bordeaux, etc
Thanks for the reply!
Our good news! We’ve booked A route via San Sebastian to Salamanca using the suggested Hendaye→ San Sebastian hop. Managed to get reservations for them all too, using b-europe.com and interrail.eu.
Quite a bit of admin, but good to know we’re al set.
Tomorrow we’ll attempt the journey back.
Thanks again for all your help.
Sym
Great news ! Which train were you able to book within Spain ? Most trains I could see were unavailable… (annoyingly)
Do not hesitate to come back for further questions !
As a suggestion for the return : you could go to Barcelona in a day, spend the night and following morning there before taking a night train from the border to Paris.
@thibcabe We ended up going Sebastian>Segovia>Salamanca...was the only route we could find that had reservations.
On the return we’re looking at your suggestion of Barcelona and then on to Latour-de-Carol and the sleeper to Paris (before Eurostar on the morning we arrive in Paris). It seems that I can’t make a reservation on the interrail.eu site: is this because they’re fully booked, or is there another trick to getting a night-train booking? (Just within France)
(Sorry, the sleeper would arrive in Paris on the morning of the 30th so we’d leave Paris in the afternoon of the 30th)
Thanks for the answer !
There are spaces left so I don't know what's going on this time. You should be able to book through https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish (change from seats to couchettes).
Let me know if it doesn't work. You can also call SNCF to save the 4€ booking fee : https://www.sncf-voyageurs.com/en/customer-service/contact-us/telephone/
Oh you’re right! I’ve found that I needed to press “later” and “earlier” a few times on b-europe for the trains to turn up. Initially it said there were no matches, but prodding it a bit allowed me to book. Thanks! Booked all the way back home now!
One further tip. The trains from Barcelona to Latour de Carol doesn't show in the Eurail planner, so you have to add the journey manually. Your pass is valid, and no reservations are required.
Tip, as ever, courtesy of Seat61, who also shows you where to find the timetable for these Spanish trains!
https://www.seat61.com/Spain.htm#London_to_Barcelona_by_overnight_couchette_train
On the sleeper, it should give you the option of a space in a 6-bed or 4-bed sleeper. The 4-bed is well worth the small extra cost, if there's availability
