I have inherited a set of Bonty Big Earl wheels
The holes where you bolt your disc onto have knackered threads
Can I do anything?

Put helicoils in?

How knackered? It's sometimes possible to just run a tap through the threads…

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retap to M6, if it's possible to get that through a rotor? (which I doubt to be fair)

Helicoil

Is there enough meat left to run a tap down and use longer screws?
Otherwise helicoil

or you could change the hub? Good excuse to learn how to build a wheel!

helicoil I hope- just need to finmd someone near Glasgow that will have something for such a small thread

Doubt there will be room for Helicoils – there are metal fillers which could be used to fill and then re tap. Or,just ream them out and go right through the flange at the back then use a locknut and bolt. You might have to file the back of the flange to get a flat surface but should be straightforward. Get stainless fasteners from the yacht chandlers in Scotland street

Unless the metal around the holes is very little then there is enough room for Helicoils. You can get them down to at least a M3 but they are fiddly to do M5 is nice and easy and straight forward so you should have no trouble getting them done. You could even buy a helidoil kit and do them your self.

helicoils that size are easily available no problem and you remove such a small amount of metal that I see to issue with installing them.

Just buy a kit and diy its not difficult

For example http://www.tooled-up.com/Product.asp?PID=152168&Referrer=froogle ( I think that is the right thread size BUT CHECK before buying)

Dome helicoils myself many times, easy. go to Specialty Fastners They will have a kit (keensert rather than helicoil probably) to suit you. Before any pedants complain about spelling that's the american method, they have lots of clients in the states.