I didn't notice a lot of difference in upgrading from 4gb to 8gb apart from using photoshop and some higher end games, so it would follow that 8 to 16 would be even less difference. £34 quid isn't an awful lot to spend though!
If you are running a 32bit version of Windows then forget it as it can't use any more RAM than you already have installed.
But if you have a 64bit version of Windows then as the others say - it might be worth it depending on what programs you are using, and how many you have running at the same time. If 'Task Manager' shows that you have little available memory then an upgrade may well be worth it.