Renew or delay mortgage deal

Hi all,

New to the forum but have used it before for some good tips and hoping someone can help now.

My 5 year fixed rate finishes in March (currently with RBS @ 5.4%) so I need to look for a new deal.
The added element is that next summer I'll be looking to move from my flat and find a house with my girlfriend - so the dilemma is do I try to find a flexible mortgage to allow me to change fairly quickly or is it better to drop onto the SVR for a few months (potentially a year depending on when we find somewhere)?

My LTV should be just under 80% so I'm hoping to get a reasonable rate, I'm just unsure if I should fix now or wait until we find somewhere.

Thanks
S