this year we have simply masses of caterpiller "nests" on 3 of our apple trees, they are like a grey cobweb attached to the leaves, some have hatched and there are simply thousands of them, any ideas about getting rid of them

If you can open up the web and spray them with soapy water that will kill them. If you leave the web on the soap just makes the web wet and doesn't seem to get to the caterpillers.
Tent caterpillers tend to go in a 7 to 10 year cycle. Some years are just much worse than others.

Are tent caterpillars the red patterned, hairy ones that my local council wants to know about and will come out and remove asap if you notify them cos they shed spines or fibres that can irritate our chests?

Not sure about that Mims but it's very likely. At least if they are not the same as our tent caterpillers then they are probably related. They are covered in hairs and have a row of white spots down their backs. On either side of the white spots there is a row of orange spots and on the outside of the orange spots is a row of blue spots.
I believe they do shed the hairs which cover them. There is a picture of the North American tent caterpillar on my photos, page 2 I think.