Hi Gareth - Thanks for getting back to me.
To answer your questions:
1. No they are not the same brand of tyres - one is Sailfish, one is Nereus (same brand group), one Bridgeston, and Potenza.
2. Yes, the tyre pressure seems to match the recommended on all four tyres.
3. No - I have not yet, I will check this tomorrow.
4. No - There is no vibration felt when braking.
5. Yes - bought through a dealer, I also have 6 months warranty but I am not sure to what extent they cover. I was thinking of bringing the problem to them but I was looking to know firstly what might be the problem so they don't try to fob me off!
Thanks for your help!