Basically I've had Mitch since 8th December 2010, so nearly 5 weeks now. When I trialled him his hooves were in need of being trimmed, he had 3 shoes on, already lost the 4th. By the time I finished riding him he had lost another one (come loose at a trot). He didn't show any signs of being lame or reluctant while I trialled him, nor for the first 3 weeks of being at my house. He came to me all trimmed up and barefoot, but farrier had trimmed hooves too short so he was a bit tender (owner had notified me of this before I picked him up).
2 weeks ago I got on and he was noticeably sore, I thought perhaps from the ground being so hard, so I called out the farrier and had him put front shoes on for me.
On trimming one of the front hooves my farrier pointed out there was a big bruise on his hoof, big enough to have been two joined together, and said it had been there for some time. He cut what he could out of the bruise, but had to leave some in there as it was to deep in his heel to cut out comfortably. I rode him lightly a few days later and he is perfectly fine at walk and trot, but puts up a fuss when I ask for a canter, and paces and shakes his head instead. He also won't turn left without a lot of coaxing (during warm up). He is a very well behaved horse for a 7yo so I put it down to him being sore and not testing me.

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I don't know anything about bruises, do they go away on their own like humans, do they need to be cut out as the hooves grow, do they stay there? Is there anything I can do to help the bruise go away faster, and is there anything I can do with him while he is sore, I'm not keen on riding a sore horse, but him being only 7, 16.1hh and a successful ex-racer I don't want to leave him out any longer than I have to.

I'd say give him a break from the riding for a bit. I bet he's being especially patient when you ride him, especially if the ground is firm or rocky. If you do ride him, ensure it is on soft ground. Is he being especially fussy on the lead or side you are asking him to canter on? There's no harm in not cantering for awhile, but with the way you discussed the bruise, I would let him have some time off.

To prevent future bruising issues, I'd suggest the purchase of EZ boots when you take him out in order to keep his feet protected.

Also, make it a hard fast rule to not ride when your horse has a shoe off. This could put them off balance and create more problems (which could also be a part of the reason he's being fussy.)

spend a few bucks and get a vet to check him out. it's the decent thing to do.
a vet can tell you what you want to know, instead of a bunch of strangers guessing what's going on.

personally, i'd pull the shoes and keep him barefoot and use EZ boots (the glove) for his front feet. I'd find a real barefoot trimmer not a farrier because lots of times, farriers do a flat trim that eventually causes lots of problems. not to mention, the garbage of his being cut short. no respectable farrier or traimmer would do that unless it was medically necessary.