John,
So I take it the engine is getting hot?
I'd start by removing the mixing pipe and the thermostat and make sure they are clear. Perhaps the easiest jobs before anything else.
There was a thread posted just before Christmas titled "Bukh DV24 overheating", or something like that. Have a read, some good info.
Mike

I have a Bukh DV20 and recently had a similar problem that turned out to be a small fish (the mechanic called it a mud skip?) trapped after the through hull fitting but before the seacock. The mechanic diagnosed the problem by removing the raw water hose from the seacock and then opening the seacock, expecting to see a small fountain but there was only a trickle until he pushed a cable tie down the bore of the seacock and partially dislodged the fish. Maybe not your problem if the flow out of the pump is "reasonable".

Based on other comments about the DV20 the next things to look at would be:
1) the "Y" pipe that comes after the pump and carries the cooling water to the block, 2 mechanics have said that these are prone to blockage.
2) the thermostat not opening and/or the housing blocked with scale and gunk.
3) build up of scale in the block (plenty of advice on the forum on how to fix).

I'm contemplating adding an Engine Guard to the block to provide early detection of overheating and low oil pressure.

Cheers,
Kinora

Thanks all, very helpful comments. removed all hoses, checked inlet and pump output. Then took hoses off Y piece and carefully poured about 200ml CLR down the pipe. Left it for 30 mins, reconnected hoses and, hey presto, 15 litres per minute flow and engine running cool.
Have owned boats with DV20 for 34 years and never had this problem before.