In the event your husband takes the part time job, the best thing to do is submit a revised income and expenditure and this will show that your household income has increased by £X per month and that that your mortgage payment has increased by £Y per month. The net effect is that your contributions would differ £X-Y.
I would go on to say that this is as opposed to not taking on the part time employment in which case your contributions would have to be £200 per month less.
I would envisage that £X-Y would be less than £200 and that in the event of this leading to the requirement to vary the terms of the arrangement, creditors would look more favourably on it as you have taken steps to attempt to increase your household income to at least part cover the increase in your mortgage payment.
I hope what I have said makes some sense
Tell it like it is.
Tell it like it is.