Hi Derek - welcome to the forum.
As Simon H. says, tasty looking bike.
Ariel 500 single engine - looks to have a standard (1954-on) cast iron barrel, do I see aluminium paint? This basic engine goes back to the 1930's (or even further), the details changed but most parts are interchangeable so ignition timing will depend on what's inside and what it runs on. The factory advice was always 5/8" BTDC for high compression engines (1/2" for low compression engines - I have got those figures the right way round, the advice goes against what you'd normally expect but that's what it is! ) but as that's a manual advance magneto you'll be able to tweak it on the lever.
It's difficult to tell from your photos but that frame looks a bit like one of the contemporary (ie early '50s) swing-arm conversions (to rigid/plunger frames) that were available before the factory swing-arm came out.
Regarding the magneto, as mentioned, many people round here will recommend Tony Cooper, I can think of at least two others that I use ('cos they're nearer to me), offhand I can't think of anyone in Chesterfield though.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)