I bought a suitcase style solar panel from Maplin's for about £50. I was without hookup for 5 or 6 days last year and managed fine. Yes, it clips to the battery. All the instructions are with it and all the necessary clips and cables. I just stood it in front of the van.
Breeze buy the biggest wattage you can afford ie 100 watt' eBay is a good source for solar panels but make sure you have a charge controller as this will stop the battery overcharging.
I paid around £125.00 for my 100watt panel and it came with a controller and all the necessary leads and fittings, one of the best buys I ever made for my caravan and have had weeks away without EHU.
Thanks for the tip spitzdog. I was looking at them last night on eBay, but was a bit confused by the varieties and charge controllers. Wasn't sure I needed one, but now know I do. Thanks.