I have a 30 year old camper with a bit of a roof leak around a roof light. I removed the light mechanism, cleaned it up and resealed it with a good quality exterior sealant. I still have a small leak in strong rain. Can anyone recommend a plastic type spray or brush on paint? I don't really want to spend £60 on some two pack mix if possible. 

Many tanks Jon

I used bitumen tape on my leaking sunroof. Not a drip in 2 years or so.

You don't specify exactly what you're trying to seal, but something like a windscreen stop leak type sealant might work, there's one called seek & seal, which is a thin liquid goes into the same route rain would take (hopefully!) & seals the leak from inside

Russ mentions windscreen sealant. By the sound of it a very runny one that runs in and finds the leak/gap.

After replacing an old fashioned (not bonded) windscreen some years ago. I got it "popped" in OK, but it kept letting rain in. I used a tube of black goo-ey windscreen sealant. Working slowly around the rubber seal, outside between seal and car body and inside betwee seal and winscreen. It takes 48hrs to go off, but worked a treat and stopped the water getting in.

The other trick if you strip and reassemble it is to have all parts as clean as you can possibly get them (and renew any rubber seals if you can).

Good luck!