Restall seat diaphragms

Hello all from Australia,

I am a classic car tragic who owns a few classics, including (on the British side) a 1962 Triumph TR4 and a 1965 Gilbern GT1800. They can be seen at www.grouseguitars.com.au/cars.htm

This posting is in search of information regarding the 'Restall' seats fitted to Gilberns (and other cars) at the time. Specifically, the rubber spring diaphragm fitted to them as the primary springing.

The driver's seat diaphragm has finally failed on my Gilbern (the car can be seen at www.grouseguitars.com.au/gilbern), which I like to think is due to age rather than my weight (!), so I am looking for a suitable source.

Early indications are that the early Mini diaphragms are the same. Any comments?

Cheers!

Pretty pretty car! can't help with the question though I wonder if an MG seat might be closer in size?

That diaphragm looks identical to the ones used on the low back seats in my Triumph GT6 mk1. Take a look here:
https://www.newtoncomm.co.uk/products/2779-mini-rubber-diapragm-seat-base-mk1-mk2-mk3-mk4-mk5/
As you thought, they're for the Mini but I understand they're the same size as the GT6's (and possibly your Gilbern).