Fuse is ruled out as it occasionally works.
Had a similar issue with a Clio we had problem is the main plug to the fan and control unit sits under the scuttle panel and dirt and grim get in there along with damp and moisture.
If you ring Renault they'll want about £180 - £300 to look at and try fix.
If I was you and I don't know how mechanically minded you are or the people you know who might be able to help you want to basically:-
1.) Remove wipers
2.) Remove scuttle panel below
3.) You'll see then a black plug and the wiper motor
The black plug is a tad fiddly to get to so it is easier to remove the wiper motor but not essential but a damn site easier.
The plug will most likely and hopefully be covered in grim and damp, unplug and clean it up carefully, spray some WD40 on the connections as well and reconnect.
Turn on the heaters and they should then start working again.
Thats the simple guide there is a few bits you'll need to unscrew but you'll see that when it comes to it, I'm based in Somersham as well so if needs be and I'm around I'm happy to assist and help it's not a long just just a fiddly and patient one.
Anyway thats what I'd try first 95% of the time thats the issue it's a design flaw
If it's the motor that needs removing it's the same process and from memory I think it's just 2 x 6mm bolts to remove it. If what I said above doesn't work I'd suggest going to a scrappy and nabing a motor from a Clio there on the cheap / free