[byzantian]

Thanks for your extensive reply. I would only add that in addition to the benefits you cite for the 0W/30, slightly better fuel economy might result from the lower friction?? With 33,000 miles on the clock, I think I will stick to Citroen UK's recommendation and have in the meantime ordered from Amazon 1L of Total Quartz Energy 0/30 for topping up.
For anyone interested, I have attached a PDF scan of what Citroen gave me as a guide.

 

[byzantian]

Thanks...never realised Aldi/Lidl even sold engine oil but will keep a look out.