How do you suggest that we prevent this disease in the caravan's water pipes after it has stood over the Winter?

There are several different manufacturers of sterilising tablets/powder for caravans. Not sure what is available in France, check with your local caravan dealer, chemist or diy store for starters. Either that or internet. Between that lot you should be able to find something.

Legionella is something not relevant to caravans,easy to look up on the 'interweb'. Whilst not a mechanical engineer I did come across it during my life in the construction industry. Legionella is about temperature and atomising or fine mist sprays of water. I think the concern here is about ensuring pipework is properly cleaned before use and there some useful responses that I shall be taking on board.

We last used ours in early Feb, and we did use the on-board water tank. I drained it fully as we packed up to leave, but I didn't go to any great lengths to blow out the pipes. We're using her again this weekend and I was just going to flush the pipes when we set up. Should I be using sterilizing tabs/fluid?

Like you I only opened and drained the system last time I used ours. I've ordered some Puriclean tabs from Amazon to clean 'my pipes' as suggested.

As mentioned there is not a real risk from from Legionella in a caravan. Risk is increased by the use of shower heads etc that produce aerosols. Infection is by contaminated aerosols entering the lungs. Legionella breeds on biofilms produced by other contaminants in the system e.g. bacteria/algae. Legionella is dormant below 20c and if you purge your water system (predominantly the shower head) with water above 60c you are sorted.

Beryl, i use baby sterilising tabs from the supermarket.

Dirty thing!

Many thanks for all your helpful advice, much appreciated. Dafydd and Beryl