I have just done what I should have done before posting my question above. I have a rusty old steel wheel fitted with a perished 700/25 tyre. Hence the new wheels and tyres I have coming. I fitted a 700/28-35 tube and although it was a bit of a faff getting it in the tyre once done it pumped up OK. I think the answer to my question is yes, at a push or in an emergency, but tubes the right size would obviously be better! So, out with the wallet.