It will depend a bit on what MM you are planning to do. The Kimm oops sorry OMM being at the end of October can be pretty chilly (though so can the LAMM in June - they had snow in 2004 I think!) so you tend to need a better sleeping bag or shiver more! However the key thing is to have a go so use what you can afford, beg, borrow or steal. A back packing tent of around 2kg is easier to borrow and can be spilt between the two of you, lightweight gas stoves are pretty cheap, and you can sleep on bubblewrap. A decent MM sack is around 40 quid and worth the investment - always useful for other things such as biking and long hill days especially if its not too big (25L good general purpose size). As for sleeping bag - get as good a lightweight bag as you can afford. I believe there are some some reasonable lightweight synthetics around, otherwise if its down you do need to spend the money. Cheap down is not worth it. Otherwise as a fell runner you will have the clothes and waterproofs that will do the job.