Hi Neil. Are they double pole switches? This is what Beama guide says: Double pole switching within the fixed wiring is known to produce a strange phenomenon whereby switching OFF a double pole switch supplied through an RCD can cause the RCD to trip. Single pole switching does not produce this effect, and it is known that changing over from double pole to single pole switching can overcome the problem, where such replacement is permissible and safe.
The phenomenon is explained by the fact that while capacitance between neutral and earth will exist in all installations, the earth leakage through this capacitance will be negligible due to the low (almost zero) potential between neutral and earth.When the neutral pole of a double pole switch is opened, the voltage across this capacitance will suddenly increase, with a subsequent increase in neutral to earth capacitive earth leakage currents. This increase will be at a maximum if the neutral opens first and will be aggravated by arcing at both switch contacts which will cause high frequency voltage spikes to cause even higher neutral to earth leakage currents.The effect may be aggravated further by the slow break feature of the switch often found in domestic a.c. switchgear.
Closing a double pole switch may also trip an RCD (as may closing a single pole switch), but in this case any tripping is unlikely to be caused by neutral to earth capacitance since the fast-make characteristic of the switch reduces both arcing and the time that any significant voltage might exist between the load-side neutral and earth. Any circuit, which incorporates only single pole switching, will never experience this type of problem since (under normal operating conditions) the neutral to earth voltage is so low that the neutral to earth capacitive earth leakage is negligible.
The problem appears to occur most frequently in installations where a separate consumer unit and RCD are installed.The opening of the consumer unit switch will allow a higher voltage (possibly 230V) to appear suddenly across the neutral to earth capacitance of the whole installation.This can result in sufficient earth fault current to trip the RCD. (Sorry just re-read the question, I was referring to switching on and off) 

Seconded on the switched neutral or double pole with neutral breaking early.You do not say if it trips off on switch-on or switch -off, but that can be a clue.

Some RCDs/RCBOs ride through these transients

Mike.

Any circuit, which incorporates only single pole switching, will never experience this type of problem since (under normal operating conditions) the neutral to earth voltage is so low that the neutral to earth capacitive earth leakage is negligible.

There seems to be an assumption there that single pole switching in the N isn't used. While it's usually not permitted in fixed wiring (for one exception see 463.1.2) it is relatively common in appliances. Most modern appliances need to be safe when connected reverse polarity (to cope with reversible continental plugs/sockets  - i.e. they can make no assumptions about the blue wire being close to 0V) and often where there are two switching systems (e.g. an on/off switch and a thermal cut-out) it's quite convenient to put one in each pole.

Often problems with DP (or just N) switching is really just exposing earth leakage problems which are hidden when half the appliance's wiring is kept close to 0V.

   - Andy.

This issue has been reported to me across numerous installations and as such I am not yet clear on whether the switches are single or double pole as the OEM of the cooker hood varies

This issue has been reported to me across numerous installations and as such I am not yet clear on whether the switches are single or double pole as the OEM of the cooker hood varies