Hi cemlyn and welcome to the forum.
As your IP for an early settlement figure, which should be based upon the value of outstanding monthly contributions, with a discount for his fees in the fourth and fifth year which will no longer be required.
If your father in law is unable to offer a payment at this level, then creditors may be prepared to accept a lower offer on the basis that you will be unable to continue your payments when you have a child. But as that has not yet happened, I do not feel that your creditors will be terribly sympathetic - although their is always the argument that a lower figure paid earlier is better than waiting for monies to be drip-fed over the next 2 1/5 years.
The main thing to bear in mind here is not to put your life on hold just because of the IVA. The IVA needs to be flexible enough to adapt to those life changes we all experience within a 5 year cycle.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk