"......I think my hand brake is marginal". What on earth does that mean in English. IMHO it means that you know your hand brake would make you fail the MOT Barrow, so go and get it fixed. Be a grown up and sort it out.
Now the situation regarding MOT test validity. The MOT is NOT a roadworthiness test. It is merely a check on a number of important issues with regard to a motor vehicle.
If you have a current MOT and submit your vehicle for a test one month minus one day before it is due then if it passes you will get issued with a 13 month MOT certificate, and so the expiry date will remain the same. However if you have it tested any earlier than that then you will get a twelve month certificate and will bring the renewal date forward. Only one MOT test and its result will be valid, and it is the latest date of the MOT being carried out. It's a bit like the army, in that you always obey the last order first.
Now in times gone by it was possible to have an early test and if it failed "lose" the certificate and continue with the old one (as if anybody would do that!). However the test is now recorded direct from the test station to the DVLA computer as we all know, and this puts you at risk if you try to ignore a fail in an earlier than scheduled test.