Many thanks for so many replies!
Just to confirm, yes its a perkins p4. Older than 1978 as it pre-dates the two letter prefix before the engine number. And yes, I meant fresh water cooled rether than raw water, my bad...
So I have lots more leads to follow now in terms of replacing or refurbing pump. And I realise that changing the engine, mounts, etc, may cost a lot more than £800 so we're prob better off paying for a refurb pump. I'm just worried that we'll run into more problems down the road and it seems so difficult to get replacement parts for an engine of its age
I'll follow up on all the pump specialists you've listed, Thank you!