Judging by my recent experience with a well respected backman in the North East, if the problems persist, and you need a second oppinion, call your vet. A simple back strain can be put right, but if the back strain is being caused by a lower leg problem, or more than a muscle strain, any amount of back treatment wont fix the cause.
Richard Auther does your area. I've used him on a few occasions and found him very good. He's a mctimony practitioner and is highly spoke of. He doesn't charge a fortune either.
Another vote for Andrea Bainbridge, does a lot of high level eventers and worked wonders on my daughter's old jumping pony who had back issues.
For general tension Dianne Ritchie has used McTimoney to great effect on our ex-racer - you can really feel the difference in him when she's worked on him. We use her regularly when he's competing.