Thank you ALL so much for the advice - I really had NO idea where to start. I will look in to the UK brokers. I think I'll try to ring initially, I'll let you know how I get on.
No, my lender will definitely not extend my permission to let under a residential mortgage. My LTV is less than 75% so I hope I'll be able to arrange something.
Get Me Down Under - your problem is complicated by your need to release equity, otherwise you could arrange another fixed rate for say 5 years and then ask permission to let - I believe most will grant this (don't say you're emigrating, just say being seconded for job position) for the term of the mortgage arrangement, this will give you time to decide what to ultimately do with your property. I will also look into BtL via a broker, I'll let you know how I get on, if you have any luck please PM me.
Many thanks all.