Tested it again yesterday and takes a good while to get really hot - about 15 mins.

I took the fuse out of the fans and let it go up a bit more than usual... then waited a good 5 mins to watch the gauge drop ever so slowly... then opened the bonnet and it dropped at three times the speed.

I also have a direct capillary gauge by the way - the sender is in the block so I was looking at that gauge too.

Not sure if this helps much?

Andy