So to summarise, you wanted help to repair a (foreign to this forum) water cylinder that you found no information on because it's probably been discontinued and you couldn't provide pictures or info of the actual element/stat that's...
At your age and experience a Fast track course is probably your best bet. Give them a call to get a feel, talk to trainers, visit training center and ask to attend a class for 10 mins to make sure it isn't a sham.
You're contacting a UK forum about a French made cylinder, nothing is obvious.
Try Atlantic, they're a large multinational and probably have a network of approved installers/repairers.
With no heat loss it would take approx 2 weeks to raise the temp of 5000L of water from 30 to 60 degrees using the probable 2kw for 6hrs a day you'd get from your array during the winter. To reduce the temp of store from 60 to 30...
Dual heated as in dual element and no picture?
Replace both but honestly try harder. There must be a social media presence for expats in your part of France that could advise...
100m2 isn't too large a property and your heat loss is 8KW?
The equivalent of two floors of 10x5m so area of an average semi so I'd say it isn't that well insulated.
Well insulated for that floor area would be 5kw or less.
Heat pump...
This has been discussed several times on this forum over the years. Use the 'search' box at the top right of the page and look for things like 'how much to charge', 'quotations' 'estimates', etc.
IIRC, the consensus is to have an...