Mr. T,
Fairy Light bulbs are normally wired in parallel (if one bulb blows, the whole lot don't go out).
As long as the total cable length is suitable for taking the total bulb current load (I assume you'll have about 50 x 40W bulbs!) Then the cable 'shouldn't' fuse into a molten sparking mess.
50 bulbs @ 40W = 2000W = 2kW @ 240V = 8.5A (say 10A)
Use a purpose made cable and weatherproof bulb holders available from electrical wholesalers.
If you black out MB and end up in the hospital, don't quote me

However, if you're going to daisy chain some cheapo Christmas lights, note that each string of lights will have it's cable sized to handle the load of ONLY the lights on that string.