I'm making an assumption, because it's Chesterfield, but it looks like it might be one of @d8mok 's. If it is he'll be upfront about it. As a beater for £50, why not?
I've ridden a lot of aluminium frames with varying degrees of cracks as our cargo bikes were very prone to them!
With this in mind I can't see any issue with this one at all. Not a load bearing area and probably the result of too long a post being fitted in the past, with too little in the frame.
Stick a nice long post in with plenty in the tube and job done. Bargain price I reckon