Daz1971, the type of aircraft used for a particular flight next year and the schedules of flights can change considerably between now and then.
What I can tell you is that this year Monarch will have a flight that goes from Manchester to Mombassa via Vienna, and Basel I believe on alternate weeks, and it will be operated by a B767. However, as Monarch's B767 will be returning to its owner, Thomas Cook, at the end of winter 2009/2010, if Monarch are still operating this flight next year then it is most likely to go on either the A330 or A300.
I am afraid it is unlikely you will be able to get much more information than this as many airlines do not have their summer schedules for the following year finalised until the winter. Regardless of who you book with though I am sure a suitable alternative will be offered to you should the timetable or routing change. If you are told that your flight will now depart from LGW instead of MAN then it may be worth asking your travel agent to discount the price of your holiday the difference it will cost you to get down to LGW. Good luck!