You need to bring the arms in from the bridge, and also you need to adjust the angle of bend at the tip of the arms. It's bad practise to adjust the frames from the hinge on plastic frames, however, tightening the screws will also be of some benefit. Generally a good opticians will do this free of charge. Naturally whilst you're waiting they'll try to sell you something.
Tilbury is not too far from Lakeside. If you pop into Optical Express (larger store) or Vision Express, they will be able to do it.
What you can do at home: wash your face regularly if you suffer from oily skin. Clean your glasses with an anti-static cleaning spray. This will increase friction and stop slippage.