I'm not sure I agree. A honouring of the guarantee would have given you back a repaired frame, rather than an adapted frame. I don't think £65 would pay for the cost of a repair. However, I am pleased but surprised that they have offered any money at all, and I think it is commendable on their part.
I've only twice had a frame with problems, and that was a Planet X cheapo Kaffenback steel frame. On the first frame I couldn't get a back wheel in with a 12t (or higher) smallest sprocket because there was no clearance. A whole batch had this fault. They replaced it at their expense, with a few freebies thrown in for my trouble.
The frame they replaced it with was the newer sand colour, and I bought a set of carbon canti forks for it (at reduced price again). There was an issue that was a bit more esoteric in that I felt that the steering was weird and the bike had something of a mind of its own. They replaced it without quibble (although they did return it to a bike shop they used at the time before they had their own workshop, and the bikeshop agreed with my complaint)
They picked the bike frames up from me, and the replacements were delivered within two weeks.