Hi jumpman,
You can start with any sized bank. Obviously the bigger your starting bank the quicker you will make money. However; always check the T&C’s on offers because you can do most of them with smaller stakes. For example the Bet365 Bet £50 Get £50 in-play can be done with £5. A smaller bank may mean grinding out small wins but your bank will grow quickly. There’s plenty of offers on the calendar at the moment.
Don’t be afraid to start with any sized bank just make sure all your figures are correct and that you have the funds to lay the bets back on the exchanges.