Potential problem with the tenant

Other

I have a long term tenant who, I have always gone over and above to look after and treat fairly, they have decided to use the loft space in a flat for storage, and covered the rafters old cupboard doors and put a tent in the middle of the loft with all sorts of bits and bobs ( they seem to collet junk of the street)
This is crushing the new insulation and I'm sure it tis not fire retardant.
The have been with me 6-years and seem to be hoarding, the also covered the terrace full of items they pick up second hand, bit of wood and chimney pots, but to name a few.
I warned them about the weight and fire risks... but to no avail.
I asked the to clear the loft as it is damaging the insulation and is not fire safe and not safe to walk on and they have refused, sighting irrational accusations that it protects them from the cold and rain! _ It seems they are trying to get out of the fixed term agreement they have just signed last month with 10 months to run.
(it is not in the AST that they can use this space, but they say I said they could I don't mind but its should be boarded out properly.
Anyway, I think the best bet would for us to go our separate ways...
We have come to an agreement (their suggestion) that they would like to give notice and move when they find a new place. they are DSS and have 4-kids and one on the way so they need a bigger space.
I have no objection, but I would like to be done in he right and legal way.
They want to leave if and when they find a place, and as she is about to have a baby, this may not be straight away.
I would like:
I would like two-months notice. As I live aboard.
A guarantee that they will take all the items they have collected over the years - as it may be the case that they will just leave it.
in turn I will release them from the AST.
Can I advise them that if they would like a good reference from me then they should follow the terms of the AST and clear everything out? like and idiot I did not do a proper inventory , so I don't know where I stand...
Any opinions on the best way to deal with this - so I don't end up breaching the law as ' landlords seem to be enemy number 1 of society nowadays?'
Thanks in advance.