Colin Johnson said:

My instincts tell me the fence should be earthed

I'd say no need to earth the fence - as it shouldn't be an exposed-conductive-part (if the fittings are class I they should be earthed themselves anyway (and if Class II it shouldn't matter). Hopefully your wiring system won't be liable to directly make the fence live if basic insulation failed (e.g. in conduit or sheathed/armoured cables).

The fence might want to be bonded though, as an extraneous-conductive-part. It's not clear cut though - bonding in general is no longer mandatory outdoors these days (411.3.1.2), and there has been a long standing regulation for outdoor lighting that "A metallic structure (such as a fence, grid, etc.), which is in the proximity of but is not part of the outdoor lighting installation need not be connected to the main earthing terminal" (714.411.3.1.2). We can debate whether attaching lights to a fence makes the fence part of the lighting installation, or is merely a support for it. Other design considerations might suggest that bonding might be a good idea though - e.g. to act as by-pass for relatively thin c.p.c.s if large currents might sometimes be expected (e.g. from diverted N currents). Or might be a bad idea (due to touchable potential differences - e.g. another fence within reach...). Often there's no one simple right answer, just a least worst.

   - Andy.

Colin Johnson said:

My instincts tell me the fence should be earthed

I'd say no need to earth the fence - as it shouldn't be an exposed-conductive-part (if the fittings are class I they should be earthed themselves anyway (and if Class II it shouldn't matter). Hopefully your wiring system won't be liable to directly make the fence live if basic insulation failed (e.g. in conduit or sheathed/armoured cables).

The fence might want to be bonded though, as an extraneous-conductive-part. It's not clear cut though - bonding in general is no longer mandatory outdoors these days (411.3.1.2), and there has been a long standing regulation for outdoor lighting that "A metallic structure (such as a fence, grid, etc.), which is in the proximity of but is not part of the outdoor lighting installation need not be connected to the main earthing terminal" (714.411.3.1.2). We can debate whether attaching lights to a fence makes the fence part of the lighting installation, or is merely a support for it. Other design considerations might suggest that bonding might be a good idea though - e.g. to act as by-pass for relatively thin c.p.c.s if large currents might sometimes be expected (e.g. from diverted N currents). Or might be a bad idea (due to touchable potential differences - e.g. another fence within reach...). Often there's no one simple right answer, just a least worst.

   - Andy.