David French appears to be a known artist and I saw some other racing pictures of his online. I did see a page of greyhound drawings too so perhaps it's animals in general rather than horses in particular. I think this is
your manYour best bet would be to talk to any of the Horse Racing Art specialists so try: Newmarket Gallery, 6 Rous Road, Newmarket, CB8 8DL Phone: 01638 661183 for starters. I'd suggest also checking whether you have a print or an original. An original would obviously be worth more and most auctioneers would be able to access online sales records for prices.
David Elsworth is based at Egerton House Stables, Newmarket, Suffolk. I would imagine he has plenty of DO memorabilia though :)