Citroen Xsara Picasso backfiring

Could also be caused by timing being out .

Or the exhaust manifold!
when leaking it draws fresh air in and the hot exhaust gasses detonate it.
Or excess carbon build up in the combustion chamber.
There's quite a few possible causes.
I'd start with a smoke test of the induction system.

many thanks for the quick response. new timing belt recently fitted but will re- check

Get an exhaust gas check done at your local MOT station and post the results.

problem now solved. according to the service history ( so called)quick fit auto services did the servicing on it for the past three years. during that time they had done work on intake manifold and exhaust front pipe ,so that rang alarm bells. removed intake manifold .NO GASKETS!!!! just sealant. two 10 mm nuts missing so sorted that mess out and then went to exhaust front pipe / manifold. two studs snapped . so thoroughly enjoyed sorting that out .runs fine now. many thanks for the very sound advice,,,,,ron.