Earth the neutral on the generator side

You may not earth the suppliers neutral so you need to ensure that you have a 4 pole ATS switching if you are not taking full load of the transformer , or if you don't own the transformer.

Pasted the relevant reg below


6.1.6 The neutral conductor shall not be connected direct to earth or to the
earth continuity conductor on the load side of the point of control except as
allowed in 7.16.4

3.56
point of control
point at which a consumer can, on or in any premises, switch off the electrical
installation from the electricity supplied from the point of supply
3.58
point of supply
point at which a supplier supplies electricity to any premises

7.16.4.6 A TN-S system shall not be converted to a TN-C system.