If it continues to act up, it's likely the cluster or either one of the two level senders. Suggest a VAG Scan to see if there are any level sender DTC's. Know that even if a level sender reports a DTC, it could be the cluster electronics causing it and sender may in fact be OK.
Your described symptom suggests it is the cluster and if so, one option is to consider Module Master as they competently repair several faults that can occur in MK1 TT clusters.
You can actually remove and open up the cluster but there are a few delicate plastic pins to handle carefully. Also when reconnecting the clear plastic front to the main housing, caution with the fuel and temp needles as they can get pinched between the two halves of the cluster shell because there is no mechanical stop.
This TT Cluster had erratic Fuel and Temp gauges and was not a simple fix as initially hoped. It was successfuly repaired by Modulemaster.