Yes, we could have flown over a VOR to update position, and did consider that, but at the time we were not sure what the navigation problem was and we were in the London TMA at a busy time.
So rather than mess about, and potentially cause problems to others, we turned back.
Back on the stand we realised the mistake. The aircraft had been wrongly told it was 40nm or so further south, so all the DME and VORs it could see were in the "wrong" place.
I can't remember now why GPS did not override my input or at least flag it up, but not all our aircraft had GPS supplemental originally, back in the day.