My V50 has just had new battery, all has been fine for a few weeks, now when I try and start it all the lights go out and nothing happens.

I have to open bonnet and touch the battery terminals, they spark or crackle and the car will then start?

Any idea whats wrong here?

 

sounds like you need to retighten your battery cables. Since the battery is new it appears the installer didn't finish the job properly. What I suggest is pull off the positive then the negative, inspect the ends of the cable and use a cleaner brush to score up the inside of the cable terminals, install the positive and tighten then the negative, spray both with a commercial battery post sealer to keep any corrosion/salts build up. If you see any bubbles or melting on the insulation of the cables, consider repairing or replacing.