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The M40i decided to ask me for a "top up" of coolant last week whilst away.

No temp issues for that week message disappeared. Has popped up again today.

What are peoples thoughts on topping up - or should I be looking to replace/refill to avoid mixing. If ok to throw some in, will the local halfords be good enough or should I wait til I'm back and get to BMW?

Halfords stuff is fine, as long as it is the same basic 'make up' as in there. You can check on their website by putting the reg number in, although I get a second opinion from a bit of interweb searching.

More importantly, you need to ask yourself why it needs topping up? It has to be going somewhere to need refilling. Check the reservoir cap is tightened properly and that the seal is intact and good. Check the engine bay for any signs of leaks or weeps.
I have two BMs with extremely stiff reservoir caps (so stiff they often need pipe grips on to move them). My B48 was low on coolant when I got it (despite a 5,000 point check by the supplying dealer! ). I found that the cap hadn't been tightened up properly as it was so stiff, so coolant was gradually evaporating.

BMW coolant is not expensive about £12 for 1.5L concentrated.
I replaced mine completely with BMW on an N52 post water pump and resevoir change.
I checked for leaks / flushed with plain water before putting in BMW coolant.
I have heard / read that Prestone is mixable but not woth messing about IMO