Check that the reverse selector knob on the gearchange lever is in fact spring loaded. If it's not then the spring which controls the non-entry through the reverse gate when selecting those forward gears is almost certainly broken. It's a bit of a fiddle to replace it and especially so if you have to make your own as I did . . . . surgical instruments are a big help
I'm not aware if they are available as spares . . does anyone know? If not I made two at the same time and the extra one could be "photographed" and this sent to anyone interested, along with dimensions.
The main reason I made two springs is because they are so dreadfully unreliable . . . the original one only lasted 50 years! . . . bloody terrible, I reckon! I thought I'd email the actual Jowett Factory and make a claim under warranty, but I can't find their website, nor find their contact details in the Yellow Pages. Pretty poor really . . how do they expect to retain satisfied customers these days without a Web presence!
But, fortunately, there are other fine folk who have ensured there is still many references on the Web to that fine Jowett Marque.