Not had to have one refurbished but the chap who fitted out my last one used to work for Whittakers, I contacted him recently for a price to fit out a living on a new build. For a basic living he was charging around the £1600 mark.
As for costs I guess it is a bit of a "how long is a piece of string" question as it will depend on how flash you want to go, what appliances you need to buy etc.
Quite flash but not OTT just had a quote of £7,000 just for the woodwork!! THought we would set aside a budget of about 6K to complete it but its just a joke!!!
my friend had hers done about 5 years ago, it was from scratch though, cost 8k but well worth the money. Was a 7.5 tonne Iveco.
Chap who did it was in Durham though, so maybe a bit too far from you.
I dont know where in the country you are but try Newbert horse boxes Ive seen loads of their livings and with them for 6K you would get exactly what you are wanting PM me if you want there number.
Steve Senior is your man (Horsebox Services), works from home and fits out trucks from Alexanders and Select, his numbers on Horsebox world www.horseboxworld.com hes very competative and just done Andrews Bowen Surfaces truck, fab job!