ok for anyone else who wants to know this here is the info i have so far:
apparently Bull Motif and ESM can both get custom looms made up for you: you specify exactly what you need including how long each extra wire needs to be, and they get it made up for you

I haven't found out prices yet.
or you can make up your own, using an existing loom and adding in the extra wires, or just using all new wires, and send it to Autosparks to be braided: this costs £1.42 per foot, plus carriage, plus vat. In my case I estimate that my loom would need 31 feet of braiding, so that would cost £44.95 plus carriage plus vat (ie £52.82 plus vat). Autosparks are incredibly knowledgeable and helpful, they do the braiding in black plus tracers ("flecks") of any colour in any pattern you like. They can look up the exact correct loom fleck pattern for the exact vehicle you own - ideal for anyone wanting a restoration to exactly correct original spec. Cool.
http://www.autosparks.co.uk, tel 0115 949 7211
Autosparks do standard looms for all classic cars too. Not sure how their prices compare with the usual MM suppliers.
Hope that's useful to someone, and many thanks to Jonathon for his time and advice
juliet
