Ok so if you pay a lump sum off your mortgage you get a return which is guaranteed and is 4.3% (to be adjusted for the loss of interest deduction) after tax.
You definitely should look again your accountants advice.
If you have the lump sum available, the question is, what should you do with it. You need to pick the best option, leave it on deposit, invest in equities, invest in a pension, reduce this mortgage. Certainly a blanket suggestion not to reduce the mortgage is wrong. It may not ultimately be the best choice, but is is definitely to be considered.
For proper advice go to the makeover forum.