Totally agree with the last post, you should be able to find suitable cover here on MSM.
If you are planning to buy cover for only 5 months but believe that the trip may carry on after that time (dad's money permitting of course) then it would be advisable to check with the insurance provider what their stance is regarding extending the cover. Most will extend the policy subject to notification prior to the expiry of cover and on the understanding that there have been no claims made or pending against the insurance.
On that basis it may be worth considering taking out a slightly longer policy now rather than having to worry about whether there could be a problem extending it later.
Hope this helps.