I realise this is a dumb question but I'm not in the mood for smart arse comments.
I realised I haven't given any meter readings for a while. Normally some bloke just comes and takes them so I never know where from. I've found one, under the mains switch box but assuming that's electric. Where's the other likely yo be?
A water meter doesn't live in a box that needs a plastic key to get into it, so that might be where your gas meter is. You can probably get a key from a DIY shop.