Ivor,
Your supplement wouldn't automatically be stopped, though it would be re-assessed.
There are some income disregards for persons on CE schemes, which means that not all of the additional income you get from the scheme would be assessed against you when working out your entitlement to Mortgage Interest Supplement.
Your CWO will be able to give you more detailed information on what your likely entitlement would be if you start the scheme.
One of the qualifying criteria for Mortgage Interest Supplement is that
"(a)the amount of the mortgage interest payable by the claimant does not exceed such amount as the Executive considers reasonable to meet his or her residential and other needs"
Reasonable Interest isn't quantified, but may be compared with the Maximum Rent Limit which applies in the area.
If the above criteria is met (i.e. the interest is considered reasonable), then there is no time limit on payment of the supplement once there's an entitlement to same.
If the above criteria isn't met (i.e. interest isn't considered reasonable), the HSE can pay Mortgage Interest Supplement anyway, but can only pay for a max of 12 months.