NS&I linked certificates vs mortgage

I have a £140k mortgage at a discounted variable rate of 0.39% above BoE base rate for the next 18 months, with no overpayment penalties and an exit fee of £250. I'm a basic rate taxpayer currently paying off £1200 a month.

I have £12k to spare and have just put £3k in an instant access ISA.

I would like to keep another £3k together with the ISA for emergencies and plan to put this in an instant access savings account.

That leaves £6k which I could use to pay a chunk off the mortgage or put into NS&I indexed linked certificates for a minimum of a year. At the moment they are a better rate than my mortgage. Should I wait for 10th May to see what happens to the BoE base rate, or can anyone tell me whether the indexed linked certificates are still likely to be a good bet against my mortgage? I was particularly interested to hear from anyone who can advise on the timing of the savings and lending rates changes relative to each other.
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