The best solution would be to have a more modern, compact retail centre, including a square and a slightly larger supermarket (no more than 25% larger), with free (or refundable) parking. Shops to complement (not compete with) supermarket.
The preferred option would be to expand Spa Road shopping area to the north, into the south side of Eldon Way estate, with shops, leisure and parking, by removing the buildings between Alldays and Somerfields.
If a town square was built, consider holding a farmer's market.

More off-street car parking in town centre if shops are to be revived.
Use gardens behind shops in Spa Road, use Alldays carpark and perhaps demolish Alldays to make more parking and better access.
Perhaps underground parking.
Remove industrial buildings on south side of Eldon Way estate to give more space for retail and leisure - perhaps a town square with underground parking.
Make carpark behind library more convenient by putting a walkway through the church grounds from Southend Road to Spa Road and moving the zebra crossing to line up with it.
Use old woodyard in Main Road as carpark for isolated shops.

Develop Eldon Way estate for housing, retail and leisure. This would have to pay for the whole redevelopment of Hockley town centre.
Make sure there are enough jobs from new business in town centre to replace industry.

The Royal Mail site would be useful for retail or parking but only if they are prepared to move elsewhere in Hockley. We don't want to lose jobs.

The clinic is run down. It should be rebuilt as a small Primary Care Centre to work with existing doctors and with own parking.
A large new polyclinic in Hockley would damage the GP's surgeries and cause too much traffic.