Is it alright to put a 25mm tube in a 23mm tyre? I've just bought 23mm wide tyres to replace my current 25mm ones. Do I need to buy 23mm tubes too?
Generally inner tubes will fit a range of tyre sizes, for example 18-28mm. Because both 23mm and 25mm are standard road tyre sizes and close together, you're unlikely to have an inner tube that won't fit 23mm. 

Ideally if you can identify the inner tube, you can check on the manufacturers or retailers website. 

Failing that, give it a go. Fit the inner tube, and tyre and inflate as normal. If the inner tube is too fat, it will force the tyre away from the rim as it inflates, so after inflation check round the rim on both sides that the tyre is seated properly.