Now the service looks to have a new operator. According to this VOSA entry, it is Lords Coaches:
PB0002148/19 - NIGEL LORD T/A LORDS COACHES, 5 HOLMES LANE, BILTON, HULL, HU11 4EX
Registration Accepted by SN
Starting Point: Withernsea
Finish Point: Hull
Service Number: 173
Service Type: Normal Stopping
Effective Date: 12-SEP-2009
Other Details: Operates on Saturdays only
This seems to contradict a comment on this blog last week that the new operator would be Pearsons Coaches. That comment also indicated that the Saturday peak time 174/175 services, which serve many off the same villages as the 173, would also be taken over by Pearsons Coaches. (The 174/5 also run on weekdays on a commercial basis by EYMS).
The VOSA entry is probably correct, though the 174/5 could have been included on the same registration as the 173. Whatever the situation though, the new operator will be the only independent operator operating a registered local bus service within Hull, which is currently a Stagecoach/EYMS only zone. And if it is Lords Coaches it will be their only registered local bus service.
If anyone can confirm the new operator, and if the Saturday 174/5 service is included in the new tender, please use the comment feature.