Boardsie,
The allowance you're referring to is Mortgage Interest Supplement, and is administered through Community Welfare Officers at local Health Centres.
Because the supplement is based on the interest portion of the mortgage repayment, it can be difficult to estimate what a person would be entitled to. This would also explain the difference in entitlements between Mortgage Interest Supplement and Rent Supplement.
I suggest your family contact the local CWO, get the application forms (2 forms, one is completed by the bank/building society, the other by your father), and submit them. They will lose nothing by applying.
While at the health centre, your father should also enquire about a medical card (if he doesn't have one already), this could help with the medical expenses I'm sure he's got at this time.
If his GP has declared that your father is no longer fit for work, he may be entitled to transfer to Invalidity Pension (instead of Illness Benefit). Usually a person must be on Illness Benefit for 1 year before applying for the Invalidity, but exceptions can be made depending on the situation. The Pension is higher than the benefit, and there are some additional benefits (e.g. free travel, possible free electricity/gas allowance), so again worthwhile checking it out. Contact Illness Benefit section (the number should be on the medical certs he's getting) to make enquiries about transferring to the Invalidity Pension.