I was renting a property with Spark and Sons last year. We had numerous reasons to call them for problems and constantly found that it took 3 or more calls before they would bother to call back.
While in the house, we discovered that our property's electricity meter was also wired to the flat below us, and we were paying for their electricity. It took us months to get Spark & Sons to recognise this, including one phone call where we were told "it's not our problem".
After they recognised it, they said they'd rewire the house within a week. They haven't yet. Instead they set up a complicated arrangement whereby they reimbursed us a figure based loosely on our bills, and charged our neighbour the amount afterwards. However it turns out they had actually started taking a fixed amount extra from our neighbour's rent for the electricity, without telling us how much this was, allowing them to give us back less than they'd taken from him and profit from their mistake!
They also expressed disbelief when they found out about the problem, saying that they'd been renting the property for years without knowing. So that's years' worth of tenants being ripped off. Well done them.