Currently using an Easynet Surestream connection but have an option to move to a BT leased line or BT EFM.
My natural reaction is to avoid BT like the plague but just wondered if anyone had experience with these services?

Not used BT for leased lines and like you i’d avoid them at all costs. BT are generally cheaper than the competition but their support is poor in my experience.

I’ve used Zen leased lines and can recommend them highly