Thus far Flatford in York is the only company I have found that still makes frames - there must be others?
But the leaflet they sent me for their F91 frame does not include the Dexta on the list of applicable tractors!
However I found a reference to an older Flatford F4 frame:

This does list the Dexta.

Please could folk who have a frame on their Dexta list the make and model of the frame they have.
Any views on buying a 2nd-hand frame would be appreciated?
BTW I found an interesting doc http://www.hse.gov.uk/research/rrpdf/rr251.pdf which concludes that old hollow section safety frames keep their strength well despite being decades old, whereas newer monocoque cabs are more susceptible to weakening due to corrosion. This is because they are often made of thinner steel and may be spot welded rather than continuous hollow section. Structural deterioration is most prevalent for tractors manufactured in 1981-1985.
Also I read (eg see posts on old site) that all tractors used for agricultural purposes must be fitted with an approved frame and must also have a seat belt to keep the driver on board.
Many thanks,
Ben