Diesel

prob worthwhile checking it works OK first. The earlier type had a tendancy to sieze up if not used for a couple of weeks even if on the vehicle

May be wrong but seem to recall LDV convoy & 400 vans , Luton & pickups use the same bits. May be worth a wander round the scrappy.

What's it like at high revs? If holding back could be fuel line blockage further down maybe even in the tank, or air bleed into line from a dodgy joint or pipe How's starting? any different? Presume no excess crankcase pressure pointing to worn/stuck rings/major engine woes?

Normal oil filter has all oil flowing through so can only filter to 15 microns in the size available. Bypass filter is an extra that only allows 10-15% of the oil though at any pass (by having a smaller orifice) so will filter down to 1 micron getting all the crud out of the oil. with good ones oil doesn't even go black! = less engine wear. Think Discos and rangies have them fitted, but best ones I've come across are www Kleeoil.co.uk

while your at it stick a bypass filter in your oil line as well. Your engine will thank you!