what is that horrible thick cobweb and thousands of small catterpillars which are attacking bushes down here in wilts

Tent caterpillars - If they're coloured with brown and red and hairy, tell your local council. I can't remember what they're called officially, but the council here comes out to deal with them. If they're not those colours, then, suggested by Moongrower recently to someone else, she says to open up the tent webs and spray soapy water inside. Good luck.

And I see you're asking about tulips and feeding - yes, feed them now, assuming they've still got functioning leaves and they haven't died or shrivelled.

We get them here too. They particularly liked our spindleberry bushes at our old house. we found the best way was to prune out the twigs and branches they were attached to, remove those branches very gently and burn them immediately. Otherwise the caterpillars drop out very quickly and make for the nearest tree trunk or bush to start again.