Thanks for getting back to me.
There are some very cheap signal generators available on eBay and amazon and the amplitude of the sine wave is adjustable but is not displayed on the unit. they are called an FG-100 DDS and are available for about £20.
In theory i should be able to adjust the amplitude and set it using the RMS function on my multi-meter and this should be sufficient to perform the calibration of the volume, tone and bass as per the service manual? i know its probably very crude but given my lack of better equipment it would be better than nothing?
does this sound plausible?
Thanks again for your help,
Martyn