Thanks all for the replies!
@smartie93 - If it's a relatively easy engine to remove and take out then should it come to stripping it down, we'll probably go down the route of taking it out to do the work, as there isn't masses of room to work in the engine bay, unless it'll cause unnecessary hassle. I am actually dying to give the engine block a coat of paint as it's looking really grubby, so it might be the perfect excuse haha.
@sledgehammer - That would be a really nice easy fix if it did turn out to be the breathers, I'm actually not familiar with where they are on the Mini, so that might require a bit of looking in the haynes manual. What would be the best thing to clean them out with? I have some vague memory of someone saying to use petrol but I'd better check before I do that.
Is it worth using a torque wrench when reattaching the timing cover?
@klivins - Thanks for the tips, I do actually wonder if the timing cover will have ever been removed from it before. We have only got limited history, but for most of it's life it was owned locally by an old lady so I shouldn't imagine she did much tinkering. If after cleaning though we do take the timing cover off, I will certainly check for trueness to check it is sealing properly as it would be heartbreaking to see it start leaking again after putting in so much work haha.
@bpirie1000 - Thanks for the handy tip, I really wish we could do that but sadly she's not on the road at the moment so we actually can't do that. It looks like it might be us getting left with the mess! She's actually kept in a garage as part of a shared car park, so I'm not too sure what we'll do about the mess haha...
@Cooperman - That's certainly a relief to hear again that it shouldn't be a difficult job to change. It doesn't sound too painful from what you've described, but after we've gone and done a bit more investigation we might be back to ask a few questions on how to do it. It's not something I've done before, so I'm not really familiar with how to lock the crankshaft, how would you actually go about doing it? (Please excuse my lack of knowledge!)