TD5 misfiring in heavy rain

Another vote for the air box ...

Update: I checked the airbox and it was indeed wet under the filter, not gallons of water but enough to render my sheets of kitchen wipe useless. I wondered how this much water gets there. I checked the 'dump valve' in the pipework leading to the box and it seemed as if it had sealed itself through lack of use(?) I freed it off so I'll have to wait for it to rain heavy to see if it runs OK.