It does sound a lot like what mine does.
I headed out to change the oil, filter, and run through a full service but I've found thick oil/grease looks to be seeping out of the inlet manifold gasket. Coupled with the gunk in the radiator, and I think I may have bigger problems.
At the moment I'm pondering taking the heads and the inlet manifold off to take a look, change the important gaskets and look for problems. Unfortunately at least two of the head studs are obscured by the centre console metalwork and engine out would be a big job that I'm not set up for at the moment.
Assuming its the normal "senior" oil filter, FBW-P52 from inlinefilters.co.uk fits. I don't know on the nipple, but the filters are standard for a number of different vehicles so I'd imagine spares will be easy enough to come by.
If you get any ideas through other means, please share them - perhaps we can put our heads together!