Leaking heater pipes

[Luckyone]

I didn't want to disturb the heater connections when I rebuild the car as they were fine, but there was no way I could get the rusty pipes off the other end without cutting the old rubber pipes. Now (many years later!) I just can't get the new(ish) rubber pipe to seal.
I keep trying something different & thinking its ok, I did a 500 mile trip with no leaking last time. Then left it in the garage for a few weeks to find the picture above. It seems to leak more when its cold!

The blue tissue is there to show which side is leaking (brown paper catches the rest). It's generally the heater side, but after the rolling road session it was the engine side.

I tried slip rings under the jubilee clamps.

I cleaned the brass pipes out of the heater till they were shining a couple of times.

All I can think now is that I'm sure the old rubber pipes had 15mm printed on them. These pipes were the only ones not included in the silicon hose kit so I got the ones in the picture from my local motor parts place, the guy just looked confused when I asked for 15mm & said these would be the same. I'm wandering if it really is now. Looking on eBay I see some people sell 16mm & some 15mm.

Are there actually two sizes of rubber pipes 15mm & 16mm or is it just different interpretations of 5/8 as the motor parts guy implied.

Of course I have heard of the heater its self leaking, but I think that is generally from inside the heater, I can see drips forming round the pipes when the tissue is not there.