"These are GU10, as this relates to the base of the bulb. Hugely expensive compared to standard sized bulbs these hi-spot variants are difficult to find, and no more expensive from screwfix than other emporia - when you can track these suckers down. But they do what they do. I think they're only available from sylvania, would be great if there were low energy alternatives."
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Review 2 for Sylvania GU10 Hi-Spot Mains Voltage Halogen Lamp 75W Pack of 5
"Sold as GU10 Standard Lamps on Page 253 These are not GU10 light bulbs GU10 bulbs are 50mm dia These are GU20 bulbs and are 63mm dia They do not fit in GU10 light fitting The base fitting is correct but not the dia"