OK thanks for your swift reply, much appreciated.
I'm going with the rubber inserts for now, its a cost over use thing. Basically I have one return journey to make to France with a caravan: The outfit sags ever so slightly too much for me to risk over the ferry ramps etc. I tow in England and it's okay but it's only just okay...so I'm being a little over cautious maybe, but if the rubber inserts give me half an inch I'll be a little more comfortable. So, for £30 im giving them a go versus buying the auxilliary springs at £160 and getting someone to fit them for another £100. I dont mind having a go putting the rubbers in, but I'm not up for undoing shocks/taking springs off etc.