If your girlfriend's income was taken into account when the IVA was proposed, and you were effectively treated as a couple, then there will be no problem in using your overtime to negate her loss of earnings. If, however, you proposed effectively as a single person, and you have the requirement to pay 50% of any additional earnings into the IVA, this could cause some problems which you will need to discuss with your IP to find out what view they would take. Personally, I would allow a set-off for a short period of time, but would expect your partner to be actively seeking new employment in the meantime, as you have no real responsibility to supplement her loss of earnings.
Regards, Melanie Giles, Insolvency Practitioner