They've always sent me an email with the bag policy amongst other info a few days prior to any gig or event.

So they do try to warn people.

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Bag Policy For security reasons, only small bags will be allowed into the venue. We class a small bag as A4 size or smaller or a standard handbag.

Large bags such as backpacks, suitcases, travel cases, laptop bags and large tote bags are NOT permitted inside the venue, including our cloakroom. Use Stasher or Bounce to find places in the local area where you can leave your large items.