I have been looking at these conversion kits on Ebay. The prices seem to vary
enormously. Why - what is the difference? Are these kits any good, as they say
the light output increases by 300%. Advice please  2013 Dynamic Santorini black
2011 XS Lago grey
2007 HLS Stornoway grey
sometimes towing an Elldis AVANTE 574.

I believe after market HIDs are an MOT failure unless they can self level.

Lighting
Products on the lens or light source that obviously reduce the light's intensity or change its colour will become a reason for failure – applies to front/rear position lamps, registration plate lamps, stop lamps, rear fog and direction indicators,

Headlight requirements are updated to take account of the particular characteristics of High Intensity Discharge (HID) lamps.

HID headlights can cause dazzle if they are dirty or aimed too high, so they are generally (if light output exceeds 2000 lumens) fitted in conjunction with headlamp cleaning and levelling systems. Where HID or LED dipped beam headlamps are fitted the tester will switch on the headlamps and check the operation of any headlamp levelling and cleaning devices fitted.

The car will fail if a headlamp levelling or cleaning device is inoperative or otherwise obviously defective.

If a headlamp bulb is not seated correctly the resulting beam pattern will be indistinct and this will result in a test fail.