Looks like the car is parked on Housing Association land, a resident has probably complained to them about it because they want more parking space if it hasn't moved for a long time.
Get in touch with the HA and find out if it is their land - likely - or council owned. If it is council they have no right to have your car moved.
If it is HA owned explain the car is registered to an HA house and has not been abandoned, just in your name instead of the tenants name and find out if it is ok to park it there because of this. Also explain you do sometimes stay over at your mum's while visiting and ask if the parking is for residents only or if visitors are allowed to use it.
If the parking is for visitors too they should not remove the car - make sure it is put on their system it is legally parked and should not be removed and make sure they notify any other companies (tow trucks) not to remove it. Get it in writing, e-mail or letter, you are legally parked so if it does get removed you have written evidence you are legally parked and not liable for paying for it and they need to pay to return it.
If it is tenant parking only then they are probably within their rights to move the car as long as they follow correct procedure (which it looks like they are following by putting the notice on the car).
We live in an HA house and there are a few off-road bays for parking. It looks like anyone can use these (HA tenants, private owners and private tenants), annoying when home owners and others from streets behind etc use them and we have to park on the road risking our door mirrors, but I think the council own the parking bit though, rather than it being the HA. If it was HA owned I'd be complaining to them about some residents using the parking.
If you are not a tenant you may be upsetting a lot of tenants by parking there long term and it might be a good idea to find somewhere else to park regardless of who owns the land.
Or, if you and your mum are willing, you can register your car in your mum's name, problem solved. It won't mean your mum owns the car, she will just be the registered keeper of the car, the V5 document states it is not proof of ownership, you having a receipt from buying the car proves ownership. It will mean all correspondence would go to your mum e.g. parking fines, speeding fines etc but you would still take responsibility for them. Something to consider.
Tbh using the space as long term parking is a bit cheeky and probably galling for the residents, especially if they can't find spaces for their own cars. Might be worth finding a garage to rent, especially if your car is not used very often.