Thanks to all for your advice.
I've gone for the Purple line Kojack in the end. I know it's an expensive option, but it would be awkward to try and get a scissor or bottle jack under the re-enforced plate on the chassis, as I have a powrtouch mover fitted in front of the wheels. I thought that if it was raining, I'd be on my belly trying to get the jack under the plate from behind the wheel; and as my local caravan centre guy said, it would be made all the more difficult as the tyre would be flat and the van closer to the ground. So I opted for the kojack. I really like the look of it. It comes with two plates which bolt into the pre drilled holes in the chassis that are for Alko's own jack. The kojack then slots under the plate. The plate would be in plain view and easy to access, no crawling around. Plus, it's hydraulic, so a couple of pumps and up she goes.
Having a puncture would be stressful enough, without getting filthy in the rain trying to get a jack in place. I'm a lazy sod and all in favour ofan easy option.
cheers
John