Many thanks again everyone. Every post is hugely appreciated :)
Loving the grasshopper idea!
Long story short:
I had a very long chat with the environmental health officer at the council, we spoke at length about all the options and I feel much better for it. Call has been logged under 'harassment'. I have decided at this point not to proceed with any further complaint although my call remains open and I have the officer's direct number.
I have logged a call with the Police in respect to anti-social behaviour.
I have been in communication with the land lady and she has been putting pressure on the lettings agent.
Two of my neighbours (one of 6 years the other of 3 years) have wrote me glowing references!
Lettings agent has requested to meet with these tenants, I have offered to also attend the meeting (offer hasn't been taken up).
Managing agent has stated that they have never had a problem with me in the past.
Upstairs have been pretty good. Since I last posted they have only been noisy in the morning once, and I am not sure whether this was deliberate or just the sound of them mating ;) - I say this as there was some banging, but it was much much quieter and went on for longer (but not especially long!)
Today - I receive a letter from the council stating that there has been a complaint for amplified music. I have spoken again to the officer I spoke to last week, and he said that they have closed that complaint for the moment. They they will have to work very very hard and build a lot of evidence against me before any action is taken, - however, he seems to realise that more importantly, they need to realise where they are in the world, and other people have every right to live their lives and make reasonable noise. No-one is entitled to 100% peace and quiet, especially in a flat :)
THANK-YOU ALL again for your support.