Glad to see that the underletting worked out well, too. I'm a bit limited for heating more than the HLT at once, but if I can sort that out it would be a possibility
I have another top tip about underletting, as Kev thinks things went smoothly.
After underletting, and turning off the HLT tap to stop any more water going into the Mash Tun, turn off the tap on the Mash Tun as well, else when you disconnect the pipe from the Mash Tun it gets a little messy