I use a product called SuperClean, an automotive de-greaser. I used to use oven cleaner, but SuperClean has some advantages for me, over oven cleaner.
One is that oven cleaners are all in aerosol cans, and they are once-and-done products; that is, you can't use a batch more than once. You spray it on the piece, it removes the paint, but when you rinse it away, it's gone. SuperClean can be used over and over again. I use glass jars of various sizes to hold a batch of the liquid, and just immerse the pieces in it. It begins at once to soften old paint, which dissolves into the solution. The longer the piece soaks, the more the paint is softened and dissolves away, but usually, in a soak of about five to ten minutes, I can remove most of the paint. I hold the piece under running water and scrub the paint away gently with an old toothbrush.
It's also relatively cheap, at about eight to nine bucks a gallon. Couple that with the fact that it can be re-used, and the cost drops dramatically compared to oven cleaner.
Another advantage is that SuperClean is not nearly as caustic as oven cleaner, whose primary active ingredient is lye. SC has lye in it, too, but it doesn't give off as strong a set of fumes as oven cleaner does. I still recommend working in a well-ventilated area and using household cleaning gloves, though I often dispense with that last one, because I don't really come in prolonged contact with the liquid.
And yet another advantage of SuperClean is outside our hobby-as a de-greaser, it can be used to clean things around the house and in the garage. I've used it on grease stains, and to clear constricted plumbing drains, as well as to clean baking dishes with stubborn baked-on material.
I don't know if you can get it in the UK. But I have read from people over there, of using brake fluid, which works the same way, especially in that you could use a batch over and over.
If nothing else, you can still use oven cleaner, too. When I used it, I used an old glass baking dish to hold the piece, then sprayed the oven cleaner over it. Wait five minutes or so, and you can start rinsing and scrubbing the paint away. It may take several passes to do.
As far as damaging the metal goes, I wouldn't worry. I don't know of any solvents we might use to remove paint, that can damage metal. That goes for de-greasers, oven cleaners, brake fluids, to mineral spirits and turpentine, or lacquer thinners. You need something on the order of a strong acid to attack the metal.
Hope that helps, and do show us pictures of your project!
Prost!
Brad
jb