I have just done some extra borrowing on our mortgage and the difference in monthly payments between the 2yr fix and 5yr fix was about a pound.... in my opinion well worth that pound for the security if rates do go up, I can't see them going down!
Cheapest total to pay 5 year fix if you are in no doubt that you are going to stay there. If you think you may be going to move consider portable mortgage, or mortgage with reducing penalties.
If ever there were a time to fix long it's now imo. Having said that every time I have fixed I have seen rates drop away afterwards! but I have always been happier knowing how much I am paying. I cannot see how rates can really be lower with inflation we are seeing now.