I have a 1993 Ducato (Talbot Express Style) with 2.5 Iveco turbo diesel and buy most of my mechanical bits from Mike Chubb of No 1 Gear, http://www.no1gear.com 01460-67000. He's very helpful and always prepared to give advice to custoimers.
I'd suggest that you give him a call and see if either he can help you himself (though he's in Somerset, not London) or is able to recommend a supplier to you. Or he may be able to sell you a good used motor for a fraction the price of a cowboy recon one if you can arrange your own fitting. Or he may even have one on the shelf that he has reconditioned himself. As I understand it (I have never visited him) he has full workshop facilities too.