Nationwide BMR and tracker, fix both or one?

bwgames

After moving house and porting a few times, I've ended up with effectively two mortgages, both with 30 years left with Nationwide:

LTV is ~ 73%.

One at Nationwide BMR, 2.5% for ~ £70k.
Second on a tracker, 3.89% + BOE for ~ £45k.

The general consensus seems to be stick with the BMR which is cheaper in the long-term.
I can see why, particularly if you've got a short term left. Is this still good advice when dealing with long terms and second mortgages?

I like the idea of a 5 year fix, and could fix the whole thing for £30 more than I'm currently paying, but that means going on to the SMR after 5 years. But, if I stick with the BMR part but fix the tracker, I pay £15 less p/m currently, until the BOE rate goes up, when the BMR goes up by £40/month for every 1% increase.

Is there an alternative option I haven't thought of?

Thrugelmir

Keep the BMR sub account.

While interest rates remain low. Overpay the other sub account by whatever you are able.

As with a 30 year term you are going to pay a lot of interest. Irrespective of what interest rates actually do.