Posted: 22/07/2014 at 22:20
I had a similar situation last year, when a tomato plant grew in a compost bin.
I left it until it was big enough to handle being transplanted (it was awkward removing it from inside the bin but a flower bed may be easier for small shoots) and moved it to a 37" pot.
It produced a few tomatoes but they weren't that great. I think that was mainly down to me not knowing anything about growing them. This year I've deliberately grown several plants from seed and despite being transplanted to soil a month or two later than they should have been (circumstances) they're now doing well.
If you think you can gently lift your plant from the flowerbed without damaging the fine roots, perhaps with a kitchen fork rather than a big tool that will take a big scoop of soil out, move it to a pot about one foot across. If you don't have one and want a makeshift alternative, Aldi are currently selling plastic builders' buckets for 99p. They're just about adequate to grow a single tomato plant, but you'll have to drill drainage holed in the bottom first.
It's getting a bit late in the year, but I would try adding just four inches of compost to the bucket and plant your tomato in that (assuming it's still tiny and no proper leaves or stalk have grown yet). When it grows tall enough so there's three or four developed shoots coming 90 degrees from the main stalk, cut off the bottom couple and add more soil until there's just a couple of inches of stalk visible before the first branch. Don't over prune the shoots because the plant still needs some leaves to produce food.
Repeat this process until the bucket is full and then let the vine grow as per usual. Doing this promotes more roots which should result in better tomatoes. You can add a bamboo pole or some other support as soon as the soil is deep enough to support it, so that you don't damage wide-spreading roots later on, when you finally need to use it.
If you have any Gro-More in the house already, you could try using that until the vine starts producing flowers, since it may help speed up growth in the few months of summer we have left. Then you can use proper tomato feed. Last year, I used no feed at all and that's probably another reason the tomatoes weren't great.