how much interest would i get on £20 000?

Sadly these days - very little. Some accounts pay about 3% which will give you about £600 a year and subject to 20% tax if appropriate. Many accounts hook you but reduce the rate to about 1/2% after one year and hope you forget.

It all depends on the financial institution you have a savings account at...not all savings accounts earn the same interest.....Talk to your bank about this...

look around and compare the interst rates, remembering to read the small print. the main thing is don't invest in anything that will put your money at risk.even if they tell you its low risk, that doesn't mean they won't loss part of it.