stick or change

my mortgage is currently with the Halifax and my fixed rate finished a few years ago.
The rate has now changed to 3.9% in which i am paying just under £500 per month plus i am overpaying £200 per month.(29 years left, 6 years done)

My mortgage amount left is £100,000 and the value of my home is £135-140k

Do i stick with the halifax until the BOE decide to start increasing rates and then look for a deal or do i look for a deal now?

I have been searching the web and because my LTV is 75% the rates offered by several lenders are coming in at around 3.5%.(2yr deals)
about 4% for 5 year deals.

I have guided my house valuation based on a house 3 doors down which sold less than 3 months ago.

What do i do?

thanks