Not sure what effect the panel damping will have on an A-weighted reading as A weighting is biased to respond to the 500-10,000 Hz range.
C weighting on the other hand has a flat response generally between 32-10,000 Hz.
I know the Goodwood meter is an analogue 'needle' type and probably has both weighting settings, C is generally the default, so perhaps it had been set incorrectly? (or had been dropped!)
I have a digital meter, with A and C settings, (and accuracy of +/- 2dB at 114dB) so if you are in blatting distance of Maidstone you are welcome to pop round and we can experiment.
MI 5EVN