my 07 plate 1900 is burning oil ,im using the 1 litre top up at £10 a time,this barely lasts 3 weeks, cant find any oil leaks and there is certainly no black smoke coming from it(just recently passed very strict taxi test) ,its a taxi and I was thinking of retiring it in november,someone said you can just use cheap synthetic oil ,asda have a 2 litre for £10 ,do you think this would be wise ?

I think generally, fully synthetic if its a turbo and or if the service intervals are something vast like 18,000 miles. You pays yer money and takes yer choice. If youre burning the expensive stuff then perhaps try a thicker oil or different brand. Some oils are specially for diesels might be worth a try.
I run my relatively unstressed 1.4 petrol on Castrol gtx which is semi synthetic (£20/ 4litre).

By the correct oil from motor factor in 5L or 20L containers, will work out cheaper and best for your engine.

What mileage has the lump done out of interest

191000 miles,i was hoping to use it privately after it ceases being a taxi