This has a lovely, delicate balance. I suspect that choice of marmalade will send it more or less towards sweet and away from sour. We used an English breakfast marmalade that probably nudged it more than gently in the direction of sweet. Next time we'll up the lemon juice by at least half again.
A rare ingredient I picked up on a trip to Corfu - kumquat liquer is an island speciality. This is very clean, vibrant kumquat and stone fruit flavours, on sweeter side.