Most of the etchings that he did seem to be in the regular gallery domain.
Marlborough Fine Art and the RA did a book called Paintings and Drawings 1954-2001. This was released with an etching (edition of 150) of Head of Julia. It is quite a small print and seems to be available in the £2,500 price range upwards. This is probably the most accessible/ reasonably priced piece available. If a gallery has had one for a while then there might be room for negotiation. I picked one up back then, but I was really after the beautiful 1990 Head of Jake etching, which was around £4,000 at that time and a bit out of range. Still is!

Katiab, your first port of call would be Marlborough Gallery London  (www.marlboroughgallerylondon.com/) to test the water as to what they still have. We keep hearing about greatest living artists and he must be up there with Hockney, Hambling, Doig and so on, but is a bit less feted. Perhaps he is a quieter man.

I thought my book was unsigned, but then saw on the Belgrave St Ives Summer 2022 Show site that the book is signed at the back, rather than the front. Quite a nice revelation after 22 years! Thank you for helping bring that to my attention.