Well, to be honest, if she is cleaning her house once a week, perhaps it would be better if there was an arrangement to pay her (with your involvement). The problem is that an informal arrangement is precisely the kind of thing that then creates guilt etc. It would not be unreasonable for them to ask for money, would it? I don't see that as a sign of abuse at this stage, if she was looking for a cleaner. Are you absolutely sure they pay nothing, it sounds very odd to advertise for a cleaner and then someone come for free? Perhaps your mum has not told the truth, fearing your reaction.
The problem here is that the cleaning and the allotment do need work, so if these people aren't paid and you think it's dodgy, you need to remove them but it doesn't negate the need to get the work done. I don't find it particularly odd that these people are helping out, if they were being paid or paid in kind with vegetables.
It may be relevant they are from EE, because there is no real state care system there for most old people and so helping out/payment in kind is more normal. My MIL's sister had a carer live in her home for free in exchange for care. This is a very normal arrangement there, because people cannot afford expensive old people's homes and so bartering/small amounts of care which is cheap at home is pretty standard.
As someone else said, if you start going around quite a bit and being chatty and friendly to them but very present, if they are dodgy, they will disappear. I would just be very present, keep the power of attorney stuff handy so they know there's no possibility of rewards in wills and it may just be that they could help your parents stay in their homes.
If you get the state to get people to come in, they come in for as little as 20 min a day (that's what my 92 year old neighbour gets, 20 minutes for a full wake up, shower and get dressed) and this leaves a huge gap in care into which nice and not so nice people can then step.