Not sure if they send to the UK, and it's a German site, but look here: www.lampenwelt.de/Weihnachtsbeleuchtung/Ersatzlampen. Didn't know there were so many different kinds of Christmas light bulbs, although there are no coloured ones, it seems.
I was hoping for coloured ones, but the transparent 12 V, 1.1 W bulb would probably do. It's not clear either whether they have the bypass feature; the shunt wire isn't apparent in the photograph. Generic miniature 12 V bulbs are available from many electronic component suppliers, but none of them have the automatic bypass, which does save a lot of testing when one fails.
JoulesperCoulomb here you go then ;) www.ebay.co.uk/itm/Xmas-Fairy-Light-Spare-Bulb-Lamps-Noma-S1-12v-1-1w-Multi-coloured-Lamps-E5-Screw-/261691442074 . Became easier to find when searching for an E5 screw base.
I'd searched on eBay before (including the term E5) and hadn't found any, but it seems a few suppliers have come along since. I bought some of the above, but sadly they turned out to be 0.85 W bulbs. The good news is that I have now managed to obtain some 1 W ones.
