Also, what is the flight on the 2/3? Can you connect from another airport to the starting point or fly on somewhere else?
Can you upgrade on the flight you're taking for one leg? I was offered £49 on my last flight to/from Frankfurt.
If you've had most of your TP from business travel, £120 is a cheap way into silver which is probably the tier most worth having anyway, so I'd do it if I were you. You'd probably benefit from a 2 week extension to collect the points. Also possible to reduce costs a bit with avios.