The site of the reservoir has the same status as Devonshire Road Nature Reserve and Garthorne Road Nature Reserve - it is Urban Green Space.
Below are the related policies:
OS 7 Other Open Space
The Council will resist inappropriate development on the areas of Public Open Space (POS) or Urban Green Space (UGS), as set out in Table OS 5 and shown on the Proposals Map. Inappropriate development includes:
(a) development that would result in loss of or damage to POS or UGS; and
(b) development that adversely affects the amenity, open character or appearance of the POS or UGS through inappropriate scale.
Development that would result in loss of or damage to sites that have not been designated as either POS or UGS but are nonetheless valuable locally will also be resisted as inappropriate.
As an exception to the above some development on POS or UGS may be permitted if it comprises:
© small and unobtrusive development that is ancillary to the open space use and enjoyment of the land; or
(d) development that facilitates or enhances public access to Urban Green Space; or
(e) development that makes provision nearby for replacement open space of equal or better quality and size.
Tyson Road had no such protection and was regarded as a collection of back gardens and garages. It would be entirely inappropriate for this site to be treated in a similar way.
I look forward to wasting my Saturday morning explaining this to the developers!