Installing a gas meter box

fit one of the partly-underground ones?

It depends if it's your wall, or someone else's wall, and if they know who you are.

If it's someone else's wall, just wack it in with some mortar on a dry day and it'll be apples until the next shower.

Owain

Not enough space to get the wheelie bin past that one.

It's my parents house wall.

How about:

stick the DPM to the inner leaf of the wall with whatever you have to hand. Let that go off before...

fit the box with /no/ mortar at the top so you can pull the DPM out to form the drip tray. ISTM that's allowed by "The base and sides of the box must be fully bedded into the mortar to hold it..."

PS or, come to think of it, just stick the DPM to the top of hole...

...assuming you won't be cutting slots either side of the hole and feeding the DPM though them in order to reconcile a brickwork opening c.365mm and DPM at least 700mm wide :)

Get your brother to do it, and take the blame.

Owain

If the wall does not face towards the direction of prevailing wind and rain, then there is less of a problem.

If rain running down the wall and ingressing through the top flange and into the cavity is the problem, then you could fit a cavity tray into the next mortar course above the meter box, extending a few inches either side, and have a flap of lead or something that is dressed down and just overlaps the outer lip of the box. Just as you would above a window or any other opening.

If the wall is not exposed to driving rain, then after the mortar has set, carefully waggle the box out of the hole and refit it with a suitable sealant all around.