OK, just to finish this one off I fitted my new sidelight bulbs yesterday.
In the end I decided against LED & stuck with old school incandescent.
The BA9 bulbs are 12v 4w just as Bassfiend said, so I checked the wiring & figured I would try a set of 12v 24w.
I bought four off EBay for £4.49 so I have 2 spares.
http://www.ebay.co.uk/itm/MOTORCYCLE-MOTORBIKE-MIN...Boy what a difference

Apologies for the rubbish phone camera snaps and remember this is in daylight.
First the new 24w side lights.

Now the 24w side lights with the excellent Philips X-Treme headlight bulbs.

Importantly I double checked the wire gauge used on the sidelight circuit and it is exactly the same as that used for the headlight bulbs so I don't see any issues running the higher wattage sidelight bulbs.
NB: Corrosion & general connector fustyness was my original issue, all resolved with a Dremel & a wire brush attachment.
A bit of contact grease to keep the corrosion at bay and a gentle tweak of the earth tang and they should be good for years to come.
Hi there. Would these fit a Griffith's sidelights? Hopefully the fitment is the same!