http://www.amazon.co.uk/Slime-SL-SDS500-Tubeless-Sealant-473ml/dp/B000C11PYW

Any thoughts?

3 of my tyres hold pressure ok, for weeks, then randomly drop to about 12psi and stay there :/
The air is leaking from the outside of the valve stem, where the rubber meets the alloy. The wheels aren't corroded and are in pretty good condition

The tyres are too old to consider breaking the seal and replacing the stem in case it weakens the edges(?), and got too much tread left to think about replacing.

Is there anything that can go inside the wheel to help seal it, but not cause issues when its time to replace tyres

SLiME is not intended for use in tires losing pressure from sidewall punctures, bead leaks, damaged rims or faulty valves.

The only way to resolve is let the air out, break the bead on the outside, and plug in a new valve.

I took some wheels to a local place and they did the valves for free, i gave them a fiver for their trouble.

mine was an advisory on an MOT.

I've never had (or heard of) any bother getting tyres to seal on the bead regardless of age. In what way do you think they will be weakened?

Cheers
Davie