My friend, Crunchie, from Herefordshire uses Aaron Martin harness for his working cobs. His mother lives in Canada, where Aaron Martin are based. Crunchie said to me that he would be glad to help with sizing and fitting advice if I ever wanted to buy AM harness. He has imported from them several times and says it's pretty straight forward.
If you look on the AM website, you will see all their products. The prices are in Canadian dollars, so you will have to convert to ££s.
Keep an eye on 'Dragon Driving', there are some decent sets on there from time to time.