I know Horse Bits in Ramsbottom (Bury) have second hand saddles. There's not a lot of detail on their website so it might be better to call or email first to see if there's anything suitable.
Saddles Direct is based in Burnley. I'm at the point of desperation where I'm considering bribing my share horse's owner to take me, him, my instructor and the saddle fitter over there and trying EVERY SINGLE SADDLE they've got in stock!
Depends whereabouts you are in the NW! Some tack shops sell second hand tack, we have one here in north rode ( tales equine) but i dont know of any others in the vicinity.