to think DS's girlfriend is taking the piss and he needs legal advice

I have a son that age.
Absolutely no way I wouldn't be spelling it out to him that this situation is not good and could bring drama to his door.

OP can't tell him what to do but she 100% as a loving parent tell him her concerns and say I respect that you will make you own choices but I would be remiss if I didn't piint out the following.

If things so tits up in this situation, you cannot pretend you were not clearly warned beforehand.

I have done this a few times and said , completely up to you, but you need to ve aware of x,y, and z.

Thats what a good parent does.
It may take a while for it to sink in but likely it will percolate and land.

Normal parents only ever want the best for their children and prevent unnecessary drama in their lives.

Conversely some kids need to learn the hard way a few times.

But yet doing jobs which are often essential and which most of us make use of.

They might offer. But I wouldn't accept or expect them to work.

Most flats in England.

Well, I'm not a trad wife.

Neither am I. My husband co- hosts with me. I don't expect guests to work.

No its interfering.

The truth is you do not know anything... you are not part of the relationship, you don't know their pillow talk and in many cases people make the mistake of seeing a parent or friend as a safe space to 'vent' too when frustrated but that does NOT mean its black and white 'truth' and they want you to hate the person they love and get involved.

You are suppose to listen patiently and stay out of it unless SPECIFICALLY asked. That doesn't mean talked to it means asked. Example: 'mum I want to leave can I come stay with you until I find a place?' is an ask where you jump in and help.

Where as 'Mary is so lazy she didn't do the dishes AGAIN and never puts the milk away and theres a pile of clothes on the chair in the bedroom. This weekend she spent all Friday night at Sandra drinking Prosecco while I was working overtime, you know Sandra the insufferable gossip who drags Mary into drama' is not an 'ask' its a VENT.

If you act on it when you where not asked too with only one side of the story that they that is in reality just their mild irritants you will quickly find yourself pushed out and no longer you DS 'safe space'.

I have to be honest that in my home guests never do chores or "help" it's rude for the hosts to ask them to do so.

Husband, adult children etc I think absolutely yes you can ask - but his gf is a guest in your house. She shouldn't have to do chores imo

Most flats are leasehold, you buy use of part of the building for a set time (say 100 years) but the building itself belongs to the over all owner who is renting the land/building for that term.

Yes. She presumably doesn't have a copy of the lease, so nobody will be able to help her.

I used to run a business from my now husbands flat; it was not a breach of his lease. It would have been if I'd had large machinery outside or run a brothel, but they were the only things not allowed under our lease.

A bit concerned maybe, but I take the view he’s a grown-ass man who hopefully has a handle on the situation, and the balls to sort it if he needs to! Clearly the op doesn’t like the gf, (and I understand why, as I have already said she’s a freeloader), but the son may have a different perspective..

If he idolise her and is happy with how things are, there's really nothing you can do about it. As a parent, I'd probably try and have a not so subtle conversation with him around contraception, and the implications of inadvertently tying yourself to someone legally before you are ready for that sort of commitment, especially someone who doesn't see the importance of playing fair financially. Stress the importance of this, as if that were to happen, that's it, he will be legally bound to support her and a child for the next 18 years, and there will be nothing he can do about it. I wouldn't like my child to be in this situation, and being taking advantage of one bit, so I can see why you're not happy, but equally it's tricky as not much you can do without potentially putting a strain on your relationship with your son and pushing him away. Just make him know that your door is always open, and you're happy to give advice, be an ear to listen, if he needs that. Unwarranted advice is not going to go down well, so needs to be on his terms.

She said the girl stayed with her for a week and did nothing to help when asked and gave her son subtle put downs.
That's not jumping to conclusions, that's making a judgement based on behaviour you've witnessed.

He has a backbone and a life going on for him. His mother is jealous and a horrible future MIL

She's a lazy sponger for sure. Where did she live previously? Most standard/newerLeases contain a clause prohibiting running a business from the premises so he needs to check his.

Learnt - marrying a man in debt, spending her husband life insurance on safaris and new house, etc

It is absolutely fine believe me. My husband bought a flat when we were still bf/gf stage. I signed something to a solicitor. His mother made a dig about the financial set up. He told her to shut up. We married in a couple of months ( this is why he bought the flat) and I got pregnant. We have been together 12 years now and I leant all my MIL dirty financial secrets....she has now only reasons to blush. She lost contact with the GC also due to her being horrible to me for no reason