They come but once or twice a year. You must go armed with large plastic Ikea bags, pocketfuls of loose change and elbows sharpened to a fine point.  They are populated mainly by hugely pregnant women and very young babies, but you need the strength of an ox and the stamina of a mule to see one out from start to finish. We are, of course, talking about the endurance test that is the NCT sale, happening in a church hall near you next weekend (probably).

The NCT sale sounds great in theory – in reality nothing more fancy than a giant jumble sale, with all proceeds going to the National Childbirth Trust. It’s an opportunity to buy other mothers’ offcasts at cut-prices, thus saving you lots of pennies and the inconvenience of having to go the normal shops to buy things. And, oh, what treasures you will find there. The sales are filled with literally everything the new or expectant mother might conceivably need, all lovingly sealed up in clear plastic bags with hand-written labels. Breast pumps, baby rockers, high chairs, grobags and, of course, mountains and mountains of plastic toys, and rails and rails of clothes.

And I have, it must be said, bought some real bargains there. A Baby Bjorn sling for a mere £7 (full price £40 plus). A mini farmhouse shape sorter that my 15-month-old has played with since he was eight-months-old and shows no sign of tiring of, for the princely sum of £1. Ok some of the pieces are missing, but you can’t have everything.  I even bought some real nappies, unused by the original owner and, sadly, pretty much unused by me too (still thank God I didn’t not use them at the original eye-watering price)

But for every bargain, there is the thing you didn’t really need and your baby hated, because it only cost £4 or £3 or £2. Only you bought about 200 of them, which explains the apocalyptic-charity-shop vibe of your living room right now. It helps if you’re not too OCD about dirt too – because, boy, some of those toys and books are dirty. Normally I just set to with some kitchen towel and soapy water when I get home. But I drew the line last time at a Fisher Price toy garage, only £3.50, but complete with what looked like dried-on cat turd on the top.

And it is, of course, competitive. We’re mothers, what did you expect? So don’t even think about enthusiastically honing in on that  Mini Boden floral dress you like the look of, because at the last nanosecond a heavily pregnant woman will snatch it from right under your nose. Be noncholant, express disinterest, perhaps even disdain, then sneak it into your Ikea shopper when she’s turned to look at something else.

Remember, though, that just because it’s in the sale doesn’t mean it’s cheap. Because each individual seller sets her own prices, they are hilariously random, with some chancers charging almost what each item cost brand new (except now broken, with batteries run down) while others virtually give brilliant stuff away, so clearly desperate are they to clear space in their living room/wardrobe/attic.

Our top tips for surviving the NCT Sale?

1) Pick it up then decide if you want it – it’s each woman (and baby) for herself out there. Hesitating is for losers.

2) Then ask yourself – do I really have space in my one-bedroom flat for a fifth Noah’s Ark/plastic fairytale castle/fake electric guitar? If you don’t, put it back. Now!

3) Practice your mental arithmetic. £5 might be cheap, but if you’re buying 15 things for £5 that’s £75 for a load of dirty secondhand tat you don’t want, need or possibly even like.

4) Consider becoming a member of the NCT. You get early entry to all sales, which means that you can snap up the very best stuff in the first ten minutes or so, before the rest of the seething masses are allowed in.

5) It helps to arrive armed with a list of what you’re looking for – your brain is bound to turn to unfocused sludge once you’re dazzled by all those piles of wondrous things.

Other than that – sharpen those elbows!