Thanks mnttech. So 16kHz at the 6dB points - that is about what I thought it should be and it means that with the Tx offset by 7.5kHz, one is only, in effect, listening to one sideband. It only needs the filter to be a tad narrower than the standard 16kHz and the carrier starts slipping down the side of the passband. I think I shall do some bandwidth tests with the offending receivers. I feel a grumble to our CAA coming on!
John.