Thanks for the reply.
I'm wondering if the squared off tail and the long wings are diagnostic.
There are some 'gulls' in Ruislip, thanks to the Lido attracting some Terns.
I'm not certain of the colouration of juvenile Terns.
I would not expect terns to have that barring on the tail feathers (except perhaps for a sandwich tern - but they have black legs) and (unless my sense of scale is wrong) those feet look a bit bulky for any tern.
Looks to me like a juvenile gull of some sort