Can I use my EHIC (blue card thingy from the NHS) for medical cover in France?
Only if you're on holiday. If you live here you need to get insurance or pay into the system via work. If you're retired contact the DWP in the UK (if British) as you're entitled to the S1 form and health care (Britain receives your bill).
If you are working and in the healthcare system, there is also a fee to pay as it's not totally free. This can be inusured against (Mutuelle) to make any treatment totally free.