Turbo oil leak after replacing oil filter housing/oil cooler

Hi,

I had a water leak between the oil filter housing and engine block.  So ordered up a new filter housing/oil cooler complete and replaced it today. Refilled engine oil and coolant as per Haynes manual ie after refilling with oil only running engine on idle/tick over. But now have an oil leak at the back of the engine I think from the turbo. In replacing the oil filter housing obviously didn't need to do any work at the rear of the engine where the turbo sits. Seems to much of a coincidence to have a turbo oil leak straight after fitting the oil filter housing. Have I done something wrong or was the turbo going to leak anyway?

I used Castrol Edge 5W30 LL fully synthetic oil.