I got a box of 10 oFF eBay for £2 and its a 2 Min job

Yes, exactly. As Elekie rightly says, that's your problem!

Go to a Ford Main Dealer and buy some decent quality ones.

Well, first of all identify whether it's the brake light or the sidelight filament which is blowing. If it's the 5w sidelight filament, we have to ask if they are flickering due to, say, a corroded connection (as with the brake light filament). You could do with someone following your car to see if the lights do flicker. If it's the brake light filament blowing, then a possible cause could be the switch on the brake pedal. Check if that's loose. The most likely cause would be a corroded earth connection.