Just to update this - sourced a nearly new D5S bulb from a write-off for the bargain price of £25 all in. Working perfectly again now (and a surprise in the improved amount of light - did not realise how much the dodgy bulb was being affected). Once removed there was a small amount of condensation visible inside so presume this was the cause. BTW zero issues in changing the bulb/ballast so not sure what the fuss is about with regards to shocks/danger etc. Just twist and remove like any other bulb.

Nello said:

BTW zero issues in changing the bulb/ballast so not sure what the fuss is about with regards to shocks/danger etc. Just twist and remove like any other bulb.

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Because believe it or not some people change light bulbs (house and car) with the power still on. With a normal 12V then this is well below the SELV (safety electrical low voltage) limit. Were the bi-zenons not safety marked then despite we all know that one should always turn power off be touching anything someone won't, get a huge belt, hurt themselves and then try to sue for damages.