They wont be interested in the pour your heart out type of letter but more in the hard facts.
Do up a budget, maybe use the template on the MABS website, if this shows you with an inability to pay the full payment then hopefully they will extend the interest only. However the final figures must show you can pay the interest only, in other words if you do a budget and include your repayment of 900 p.m. and still have a shortfall after than then unless you can show how you will deal with the shortfall you could have a problem. For example lets say the budget after everything shows shortfall of 100 p.m., thats fine, you could say I will cut back on petrol, sky, phone etc to make that up, it is not too big. However if the shortfall after all expenses is say 500 p.m. then they are less likely to agree as you will be unable to make that up so they cannot be seen to agree to an unmanageable proposal. Make sure you have already decreased anything that can be cut such as Sky for example to basic package.
Send in letter outlining your proposal to continue interest only for further 6 mts (it can be reviewed again then) and enclose budget, assure them that should circumstances improve you will advise them immediately.
Alternatively make an appointment in your local UB with the Flex adviser and they have a process for dealing with this. Again make sure your budget balances.