Steve-g, first off, well done for getting one that quick!
The waiting list here is now two years.
Anyway, hold fire buying anything expensive to start with, if you allotment is anything like mine, your neighbouring allotment holders will be bending over backwards to help you out with clearing your patch.
They will be keen on you clearing the weeds down to stop them covering their plot too.
Go down make yourself know, take a big flask of tea with you 😉 and have a chat with the locals.
I wouldn't worry too much about growing anything just yet, as Mike mentioned, sort out your paths and start clearing things up.
The easy way to clear a patch is to cover it with some old carpet, which will kill off most of the weeds underneath, then you can look at removing what is left and turning your soil over.
Some good books out there, this is a good one to start with:
http://www.amazon.co.uk/Half-hour-Allotment-Royal-Horticultural-Society/dp/0711226059
One last tip, pace yourself, its all to easy to try and do everything in one go, then get bored or disillusioned with it all and give up, seen it happen her a few times, just take your time and enjoy.