Do you have a timetable in mind? Because then it's easier to give advice on reservations needed (if any).

Timetables for this journey are best checked via Deutsche Bahn.

Thanks for your reply. I will check their timetable. I would like to arrive in Budapest in the day Thursday presumably after a night train, and depart to arrive in Manchester/Warrington sometime Tuesday 27/2. 
thanks again

Several options (routes) are possible :)

Maybe you search for a connection via bahn.de :)

Manchester - London - Brussels/Amsterdam - Nighttrain to - Vienna
2nd Travelday Vienna - Budapest

a one travelday solution from Manchester all the way to Budapest would be quite hard :/
Manchester - London - Brussels - Frankfurt - Munich - Nighttrain - Budapest

Note you would need reservations for Eurostar of arround 30€ per leg (the seats for Railpass user are limited) and for the Nighttrains aswell starting from 14€ for a seat ;)

 

Another possibility might be Manchester - London - Brussels - Paris - night train to Vienna - Budapest.

The night train Paris - Vienna only runs 3x/week and I didn't check the dates.

seat61.com has about all the info you want, incl ways to do the bookings and possible pitfalls. It is not primarily aimed at passholders though.

In general the route via DE is advisable as you can, but do not need to REServe those ICE trains (just like mostly in UK). Unless you aim for the so often praised nightjet to Wien=Vienna (not daily from BRUssel)

Thanks for your help everyone