Interesting one - you are being charged 1.8%, which as you say is quite low.
In a regular saving account you can get upward of 4.00%.
Taking into account DIRT I would still say saving the money may be more of a benefit.
And with overpayments they are gone when they are made, and should you in future require funds, or whatever reason, at least the savings can give you a buffer.
I would make overpayments when the savings rate are lower than the mortgage rates.
Its all swings and roundabouts really, each would have a different opinion and only you can make the decision.