Diana G36 from my childhood.

[VALE BOY]

The Milbro Diana G36 is smaller than the Model 27 and is almost certainly an ‘updated’ Model 25 being of a similar size and power... a youth’s air rifle halfway between a junior rifle and a full-size adult rifle. There’s a possibility that a Meteor or a Falcon spring and possibly an ‘outer’ Relum Tornado spring could fit if a genuine Milbro Diana one is unavailable. 😎

[NickG]

Found what looked like a suitable spring in my bits box 3mm gauge 235mm long, popped it in and its doing around 7.5 fpe , nice and quiet , I hadn't realised how bad the trigger was as a child , nothing to compare it to I suppose , this would make a nice little garden plinker for the kids , if I had any lol, suppose it will go back in the cupboard now , will put a picture up when I get a chance .

[ggggr]

The Milbro Diana G36 is smaller than the Model 27 and is almost certainly an ‘updated’ Model 25 being of a similar size and power... a youth’s air rifle halfway between a junior rifle and a full-size adult rifle. There’s a possibility that a Meteor or a Falcon spring and possibly an ‘outer’ Relum Tornado spring could fit if a genuine Milbro Diana one is unavailable. 😎

The G36 is more or less a G27 and it uses the same mainspring as the 27/ 78/79/80.
The G34 was the one that is more or less a G25.

Im pleased the OP has found a mainspring to suit. Ive found the Diana 27 mainspring to be a bit harsh. I think I've used a Meteor one in the past (maybe a silver streak or turbo gold?).

Ive sometimes fitted Diana 25 springs in guns. I think I put one in a Webley Falcon to make an easy cocking plinker for my Niece.