Could be several things

Faulty appliance

The motor type of the appliance inline, axial and centrifugal.

Earth leakage

Supply to fan, is it a radial or Ring Final Circuit

Faulty RCBO

Location of consumer unit eg is it in the cold garage or hot loft

A RCBO protects electrical equipment from two types of faults; residual current and over current. Residual current, or Earth leakage

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

The trip occurs when switching between fan speeds and between any fan speed and off position, I hope that clarifies

It can also be a switching transient in the sense of a very brief overload - if the pick up coil in the RCD is not perfectly balanced (and real magnetic devices are good but never perfect ) then consider that even a 1% magnetic imbalance on a circuit carrying 10 amps looks like a  100mA of the kind that ought to fire the RCD  - in fact for some cheaper RCD designs this effect can be a limiting factor for inrush current handling.

I'd hope the switches are break before make if they are jumping motor windings, but you never know - if they manage a brief short then the current for the next half cycle may be much more than the normal load.

I think as the next question will be what to do about it the first thing to do is to look inside and see how they are made. It may be possible to suppress the worst effects either with series NTC (for inrush limiting) or shunt snubber (for voltage spikes) arrangements.. Or it may be that a change of supply polarity or brand of RCD is the way forward.
Mike.

Switching to a different brand of RCD might be a wise decision. I have observed that a low-cost 300 mA RCD tripped prematurely at 195 mA, whereas a more costly one tripped at a closer value of 225 mA.