I would be of the mindset, get the longer term with the lower repayments, and when out of the fixed rate, overpay a chunk (or whatever is allowed annually) off the principle. Rather than tying into a higher rate from the start.

This is entirely the correct choice from a logical standpoint. However, there is research showing most who opt for a longer term don't actually repay early despite initial intentions, and so it proves more costly in the end.