I've spoken to Latham's and their plywood isn't void free, although they do sell retail. I've also spoken to Edens and their minimum order is 20 sheets :shock: ! That'd fill my workshop and empty my piggy bank

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The search continues...
Gill
Edit: Edens told me about a firm in Wednesbury they supply called
West Midlands Plywood and gave me the company's telephone number (0121 505 0234). I called them and spoke to Trevor who was very helpful, telling me the company was newly established and often delivered to Redditch, near to where I live. He would be happy to let me have 1.5mm aircraft plywood at £17.50 per sheet (ex VAT) and it would be delivered FOC because they're in the area so often. Success

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I realise that 1.5mm is nowhere near the 3mm I was originally seeking, but I was only after 3mm because I didn't think there was a chance of getting anything thinner.