I bought the same bulbs from halfords too, but i think i'm gonna take em back and get a refund. When i put them in one way on +ve they are really impressive and bright which is what it would look like when u'd brake. However this was obviously the wrong way round, when i changed them to the correct way they didnt work but with a bit of fiddling they sort of worked but dimmer than usual and they were loose. I didnt have any problems with the 3rd brake light though. I took them out to mod them slightly, i filed down the insides of the connction pionts to seperate it further to match the OE bulbs. didnt have much luck here either. May give it one more go.
On the other hand, one of the reasons that i purchased these was to get a more brighter and red light. After i tried the LED's, i had some bulb paint from an old car left over. I painted my OE brake light bulbs red and stuck them back in. They have really made a difference! Instead of having a orangy-red brake light, they are more of a deep red and they really match the LED's of the 3rd brake light. So if your frustrated with trying to get the LED's to work give this a go.