You're looking at £3k to just convert it over to a suitable engine management (Pectel T2) and 350bhp, to get it to 400bhp would take a LOT more money, as you would also need to budget for the transmission as well, which would be around £5.5k on TOP of the final engine price.

To do it properly, you wouldn't see much change out of £12-13k for a YBP engine (but that includes EVERYTHING - engine / transmission). You could do it for around £7.5k and forget about the transmission, but you would never be able to use the full 400bhp .

Also the above price is for a T38 equipped engine producing 420-450bhp (depending on how much power you wanted).