Does anyone know what sort of thread I should buy to re-stitch my dog leads? I don't think my ordinary linen thread will be strong enough.

Ask your nearest cobbler, they may well just give you some.  If not try lookoing for plaiting thread for horses, that should do the job and comes on a card for under a quid.

I buy waxed thread like this from ebay, does the job perfectly.

M.

That looks much the same as the horse stuff I would buy

We used to use a proper old-fashioned cobbler to do repairs like that - very cheap and good, strong repairs but sadly he retired. A saddler would probably be able to repair leads for you as not much different to repairing tack.

If you want to repair them yourself then I would use  waxed thread but a machine repaire would be stronger I would imagine but beyond the scope of a domestic sewing machine.

'Wenzel' (we get it from the cobblers), and always wax it first, running it through liquid wax, or soft wax.  It preserves the thread and stops the thread fraying while being used through leather.

Thanks everyone; you've all been very helpful! :-)