Thanks everyone for yout most helpful comments. In the end I called my supplier, Heart Internet (acquired by GoDaddy in 2016) and they agreed a significant discount so they are now in line with many of their competitors. My problem is most of my clients need on-site support in setting up their email and the logistics of traveling round to reconfigure their accounts would be a headache. Once you’re tied into a reseller account it can be quite an undertaking to jump ship. I chose Heart internet several years ago because they were quite small (compared to the giants) and I liked that, but now they are part of a bigger operation. It’s just the way things go I guess. Like all BIG companies you just have to keep asking for a better deal because customer loyaly ain’t what it used to be.
PS Will certainly keep these recommendation on the radar for the future.