The best layed plans etc. The current sensor I was going to use will work down to trickle charge current I expect but will only stand 5Amp. This is OK if the charger goes straight on to maintain the battery but if it's a bit flat then the charging rate will exceed this. For now I think I'll just fil a switch next to the Anderson plug and I'll worry about an automatic system later.
I suppose I could fit a microswitch that the body of the plug trips as it's put into place.
(Just had a shower, which I use as think tank, and an idea came to me). I'll try a reed switch and a magnet bonded to the Anderson plug. This uses no power in use, produces to heat and I already have all the bits.
Unless anybody else has a good idea.