A biker tapped on my window tonight and told me that I only have a high level brake light working.

I've checked the bulbs, and I've "borrowed" a 10a fuse from elsewhere in the fuse box.  The high level one is still working, but the other 2 are not.

I know very little about wiring etc, does any one have any suggestions other than take it to a garage.

I have a long commute, on motorways which I obviously shouldn't be on with only a third of my lights working.

 

Have you checked the bulbs?

Sorry misread you have checked the bulbs!!! 

Try giving them a bang this sometimes help from the outside if so, whilst the bulbs are in place turn them so there nice n snug they tend to also loosen from time to time 

you really need to get a voltmeter on the bulb sockets if you are sure the bulbs are good.

have you proved the bulbs work by trying in another car or by putting some wires on them from the car battery.

to test on car battery -  hold the outer metal casing of bulb on the positive terminal of car battery . then use a piece of wire from the negative terminal of battery to terminal on bottom of bulb. make sure the wire is insulated (meaning it has the plastic coating on it apart from small bare bit of wire each end. ******DO NOT SHORT CIRCUIT IT******** . ******DO NOT SHORT CIRCUIT IT******** . ******DO NOT SHORT CIRCUIT IT******** . Might be safer to have 2 people, one at each end of wire in case your fingers slip if trying to do it all yourself. 

when my outer brake lights stopped working (2 blown bulbs) I found out because the police saw it and stopped me. they issued me with a VDRS notice. Better than being prosecuted but it meant that after replacing the bulbs (which was very easy) I had go to a MOT station and pay them £10 to stamp the notice to confirm fixed and then i had to send it off to the police. 

Because of this I sometimes tell people if i see theirs not working and often get glared it with an expression like mind you own f***ing business mate. funny how people take offence when you try to help them. I wish someone had told me before the police did. I know we should all check our lights more often...