Charlie,
Also, not "au-fait" with the planing speed of Shetlands, but presumably they are semi displacement similar to Orkneys ?
In which case, I would expect the boat to plane earlier than 12 knots when fitted with trim tabs. ( maybe another Shetland owner can comment on this ).
Maybe some adjustment ( ie. increase lift ) is needed ?
My 520 planes at around 5 to 6 knots with the smart tabs fitted and that is with a fair amount of weight in the back as well.