problem with roofers

I have just had 3 new flat roofs & a conservatory extended by a local firm of roofing contractors at a total cost of nearly £6,000.
The garage roof was fine, the porch roof (after 2 remedial visits) was fine, but the conservatory roof was leaking.
I retained only £500 from the final bill until they rectified the leak.
They refused to speak to me in a civilised fashion, and today broke into my back garden and started to rip off the new garage roof, and threatened to remove the porch roof saying that all the materials still belonged to them.
I phoned the police, they came to my house but said it was purely a civil matter and, because I had not paid the final balance, the roofers could remove any part of the materials used on any part of the contract.
I phoned Consumer Direct and they could not understand the advice the Police had given to me.
Is it correct that a contractor can remove all materials from the exterior of premises, and it is not even a criminal offence providing he does not actually enter my house?

update on roofing problem

Thank you Tony for your reply.
There was no provision for stage payments because the Roofers originally stated that they wanted no money,not even a deposit, until the work was completed, which they anticipated would take 2 weeks, from 11th December. They obviously had cash flow problems, so I paid them £2,000.00 on 19th December, a further £1,500.00 on 31st December, and £1,800.00 in January - leaving a balance on the original quotation of £365.00. They have also demanded nearly £200.00 for 'extras'. I paid them by cash & cheque, not credit card.

The update is that they sent me an apology by email and came back to my property today to 'repair,complete & rectify faults', with a promise of a guarantee to cover the works. I arranged for a friend to oversee them today, and to pay them the full balance. There was no way I wanted to be confronted by 3 large men + crowbar again!

It rained while they were here and the new porch roof started leaking yet again, but they said this was not there fault because the water was coming in from under tiles they had not replaced, and was running along and through the porch ceiling. Bearing in mind that I contracted them to stop the water ingress to the porch, and their solution was to alter some tiles and put on a new flat roof, I feel I have paid out money for no result - the problem is just the same.
My 2 week job has already taken 2 months.

I am told that they spent some time trying to rectify the leaking conservatory roof, but it is leaking worse than ever this evening. They have put the guttering back on the garage but forgotten the downpipe so I now have a waterfall over the garden path.

They have not returned my phone messages so I will email them tomorrow. I have also arranged for an Independent Surveyor to come on Friday.
In the meantime I need to go and by some buckets!

Thank you

Thank you Witch Consumer for your reply.
I have to admit that Civil Action is most probably the only way foward now.
I have been advised by Trading Standards to follow the procedure of 2 written letters to the Roofers, setting a time limit etc to put things right, just in case this matter ends up in Court.
Any advice is much appreciated.

Thank you

Hi Catriona
Thank you for reminding me that the limit is £5,000 in the Small Claims Court.
Apart from the missing downpipe on the garage, that part of the work appears OK. Any dispute will be over the porch and conservatory leaking roofs.
This should take my claim down below £5,000 but I am not sure if legal and other costs can be claimed over & above the £5.000 limit.
I spoke to my insurance company today and they confirmed that I do have Legal Expenses cover for breach of contract. However they added that this was dependent on their claims dept being willing to take on the matter- must be something in the small print on my policy.
I intend to wait for the surveyor coming on Friday so that I will know how to lists the faults to the Roofers in my first formal letter.

update on roofing problem

Thankfully it has not rained heavily for a few days.

I received my survey report which confirmed the problems have been caused by faulty workmanship.

I emailed the roofers, and invited them to a site meeting to discuss the Report findings/recommendations. No reply!
I sent a further email 3 days ago, briefly setting out the findings etc, and requesting they contact me by return. No reply!

I phoned my Home Insurers who said, subject to them assessing the claim form, I was covered for Legal Expenses to take them to Court. This sounds hopeful, but there is most probably something in 'the small print' to exclude my claim.

Thank you Tony for you comments.
I am away next week but will keep you updated when I return. When I have time I will write a blog, hoping that it will help others.
It has been very helpful to me knowing about this forum, and having support from the other Members.
Thank you
Jo