I have just installed a 2762 and set it up for BT Infinity FTTC, memory usage shows 87% updated the Firmware to 3.9.0-bt it's still 87%. Is this normal? All other settings are default. The cpu is at 4%.

It's normal, it typically will not raise beyond that point and will have been at 87% once DrayOS started up.

DrayOS reserves the memory upon starting and each router's firmware is designed to make good use of its RAM.