Good news all - the car is back on the road!
I decided to listen to a few of you and take the plunge to replace the expansion tank. Ordered a new OEM one (about £75 on eBay from a BMW dealer) plus some coolant, and enlisted the help of a couple of family members who are a little more mechanically minded than me.
Parts turned up last week, and managed to get it sorted on Saturday afternoon. Took us around 4 hours in the end, lots of fiddly bits - but I think the leak is sorted.
Nice feeling of satisfaction in sorting something out yourself, but I think that’s probably my limit in terms of mechanical ability!!
Anyway - have completely refreshed the coolant, and gone for a couple of runs in the car now. Yesterday was a 40 min drive whilst giving it some beans once warmed up, and so far so good! Even able to stop next to a lake for a quick photo op!
Really great to have it back on the road again. Although, I noticed another problem I’m going to need to get fixed asap… it’s not quite enough to put the car off the road again, but not great and probably something to sort out asap ideally.
I’ll post another thread about it otherwise this one could turn into a never-ending one - Thanks for the help and tips all!