guv wrote :-

So do you think I need to contact my local water authority or a
drainage company?


I am in the 'lucky' position of being the lowest property on a shared
drain - so when it blocks its me that gets the flooded garden :-(
Normal responce is DIY or get the yellow pages out but a builder once told
me to phone the council (not a council house btw) - brilliant responce, £35
ish fixed fee with a 1 week guarantee, as it usually blocks when i'm on 12
hour nights this is an excelent solution
It never blocks on my property but try telling someone that your garden is
flooded and its their problem :-(

Regards Jeff