If you know exactly when you have to pay back the money and it's a reasonable period in the future i.e. approaching 12 months (ideally much longer) then you could consider the following:-
Basically find the highest paying guaranteed rate savings account you can find, calculate how much return you will get from that over the period the £££ is available to you. Then take that income amount* and put it into a stock market tracker. If the tracker crashes and burns then you'll still end up evens but if the tracker jumps substantially then you're quids in
Depends upon the amount you have to squirrel away though, £10K would only give you a few hundred quid to play with and once charges have been taken out then might not be worth it.
An interesting proposition though for anyone looking for a punt
* not that exact amount, you need to do some maths to work out how much of the initial capital is invested in the guaranteed vehicle vs the risk vehicle (explained in the link).