the ones i looked at were mostly big firms doing concrete stuff on large refurb and construction sites or were workshops cutting granite etc.
A firm in Croydon are at least looking at the pics of the sill and will get back to me...
Here in France you go to the people who usually make gravestones for that sort of job, as they don't usually get a chance to charge money for their offcuts, so it's cheap.
Maybe worth trying in the UK too?
You could try concrete cutting companies, but I think they mostly wet cut. (hosepipe amounts of water)