Thanks, I had read this before but went back & took a closer look.
"If your income is up to twice the amount of the relevant exemption limit you may be entitled to marginal relief, however it will only be given where it is more beneficial to you than your tax credits."
Current limit is 36K, so you pay 40% marginal relief on amounts between 36K to 72K.
Wouldn't be going over the 72K so no need to enquire as to what would happen then!