Defender v8 starting & running problem

Hi I've recently bought a 1989 Defender 90 CSW with V8 petrol engine. Sounds like the previous owner had only used it occasionally and nothing strenuous. For the first couple of weeks it was starting and running fine. Last weekend we took it off-road for the first time - fairly rough terrain, but it is kitted up (including a Tuff Toys wetproofing kit). It ran fine all day and seemed reasonably OK on the 30 mile drive home. I gave it a good hose down and left it for the night. The next day it seemed reluctant to start and since then it's been been very hit and miss- either won't start at all or will start but then cuts out/runs rough. I've replace the plugs and the distibutor cap so far and sprayed WD40 all over. The leads seem pretty new. I thought that would have cured it but it started OK, I did a short run and then it refused to start again for the journey home. I'm now thinking fuel filter (but having trouble getting it it off) in case I stirred up junk in the petrol tank with the off-roading or fuel pump. Does anyone have any other suggestions before I tackle it again tomorrow?? Cheers