1.8 GTi unable to select 1st gear when hot

Having just returned from a baking hot driving in traffic weekend at Alive n V'dubbin I can suffer this glitch no more.

My 1983 A plate 1.8 gti campaign will not select 1st once it gets hot, all other gears fine with no winding or grinding. I have purchased a repair kit from VW heritage and replaced any obvious worn out parts of the linkage. I have changed the oil and put in some Molyslip and whilst the rest of the gears are silky smooth 1st is still off limits when running hot. The car has done well over 200k and the box does sound rough in reverse.

Any suggestions or is the box knackered?