"I fitted Stelvios last night, can't wait to go for a ride! I would have taken the Birdy to work today, but the forecast was for rain. They were quite challenging to fit, very tight on the rim - and I really shouldn't have been a cheapskate and reused "my original 18x1.5 tubes"

I doubt that will work, not recomended from my experience as the tyres get pulled of the rim with oversized tubes.but is you got them to work OK with 1.5 , then fine.