Thanks, Andy, Peter and Paul,
at the end of a nights’s session I can barely keep my eyes open and I have always taken a new set of flats and corresponding darks. I actually have a medical problem of not getting enough sleep which is getting to be a problem. If I was simply imaging I would’t bother, but as I do a lot of photometry the flats may have been an issue. I’ll try experimenting and see if there is any difference. Peter’s experience seems promising.