Sounds like a leak from the master cylinder to me.
In the engine bay:
check the pipes immediately connected to the master cylinder for damage/corrosion and the master cylinder itself for damage, then trace the brake pipes back from there. Also check the flexi pipes at each wheel, any leakage here should be easy to spot in dry conditions.
If you find that the brake pipes show a lot of corrosion in general it may be prudent to have all the pipes renewed, it's not as expensive as you may think for such a large job and you get piece of mind.
I personally wouldn't recommend any DIY tinkerer try bleeding brakes unless helped by someone who explicitly knows what they are doing. Too risky otherwise.
If you decide to take it to a garage it needn't be renault as any garage worth their salt should be able to sort it, defo worth getting a few quotes first.
This happened to me (spongy for 2 mins; then no brakes whatsoever!!!
It was a brake pipe routed through the interior that had a pinhole in it, resulting in a cloud of brake fluid that looked like smoke.
Glad the handbrake worked fine LOL.
I had the help of my father and we replaced the whole lot as it was an old mini (my first car) it needed done, we have since done a couple of my cars over the years and it never gets any harder or easier, just different cars.
It IS an exact science though, I'm now in my 30s and still wouldn't do it without help. This is one of only a tiny handful of jobs that I wouldn't attempt without knowledgeable practical help.