Thanks for the replies. I have started trying to eliminate the possibilities, by changing from mother culture (which I find very imprecise in terms of variable quality, uncertain quantities, etc) to DVI, and making sure the temperatures are held steady. I'm going to have another go at a cheddar later on today, so I'll keep a close eye on what happens.
Even with the DVI, I don't have an accurate way of measuring the starter as it comes in a small packet sufficient for 50 litres, and I'm using much less than that. Will be around 16 litres today. I've already used part of the packet but not sure how much so it will be a bit hit and miss.
The rennet is new, only had it around 6 weeks, but I have no idea how long it may have been in storage before I got it. Would it be possible to test the set in a small batch of milk without culture, just to eliminate the possibility that the culture is the problem? Or would that result in no set at all?
On the plus side, I cut open a red leicester I made about 3 weeks ago and although the flavour was very mild of course, it was great, so I must have done it right on that day, at least.