A PEP is what an ISA used to be. it's basically a tax free savings account - the interest grows free of tax. It's probably worth contacting the bank/building society/whoever who the PEP is with and asking the interest rate. You may be able to get a better rate elsewhere.
The other option is investing in stocks and shares, but I would only do that if you can afford to leave the money invested for at least 5 years, and stocks and shares will fluctuate over the short term, only out-performing savings over the long term (as a rule of thumb).
Finally, if you have any debts that have a higher interest rate than you're getting on the savings, you're better off paying them off.
Any more questions, I'll try to help...