This is a bit like asking how long is a piece of string. The answer will depend on how long you want to invest for, whether you have a pension and what is your attitude to risk. I'm not a financial advisor so can't offer advice but is your ISA a cash ISA or an equity (stocks & shares) ISA? If you have a cash ISA, you can open another one worth �3,000 in the next tax year, or you can take out a equity ISA up to �4000 this tax year (or a maximum of �7,000 in any tax year) which invests in unit trusts. If you want a long term investment which will grow more than a cash savings account, unit trusts are more likely to perform better but you must be prepared for the value of your money to be more volatile than cash savings.
The best thing to do is buy some money magazines and get a feel for what's available. If you use a financial advisor they will either charge you a fee for advice, or earn commission on any products they sell to you.