I'm not sure that anywhere (including the Altberg factory) offers that unless you're prepared to buy both pairs. No harm in phoning or emailing them to ask though.
Whalley Warm & Dry are Altberg Premier Retailers and may be a bit nearer for you to travel to. Their boot fitters are trained at the Altberg factory and they have an excellent reputation with customers travelling from all over the country to use their boot fitting services.
http://www.whalleyoutdoor.co.uk/boot-fitting-service/
I can personally recommend them and the best advice I can offer you is to book an appointment, go with an open mind and be prepared to be guided by the advice and expertise they offer.
Yes I was thinking of going there thanks.
I know I wouldn't ask or expect a retailer to split a par of boots, but at a factory every boot is simply added to a matching size to make a pair. Logically they simply return each boot to the appropriate size coming off the line. Then it gets matched back in.
Its because they take a pride in ensuring a proper fit that I assume they would do this?
I was unaware of any difference in my feet (beyond a bunion on one) until quite recently. Though I gather that lots of people have different size feet (maybe not a big difference). Mine is just a half size I think.