Apparently (from the web) you can find the serial number on a barcode sticker on top of the boiler. It's a long 24- or 25-digit number divided into three parts, with the last two sections showing the boiler's age. The very end of the serial number usually displays the manufacture date.
Alternatively if you can find that serial number you can phone Ideal and they will tell the date of manufacture - which doesn't necessarily tell you when it was installed, but will give an idea!
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Reply to the thread, titled "Does anyone recognise this boiler?" which is posted in Boiler Advice Forum on Plumbers Forums.