Thanks to all for your comments. The update on this is that the pick-up inside the tank is clear and the fuel filter has been changed for a Kia OE. Nothing untoward was spotted inside the old filter. The result at the moment is perfect starting and no problem under load. I know its early days, so we will give it a couple of weeks and see how it hangs before I indulge further.
The filters that were used previously were Blue Print and Crossland which cost under a tenner. I have no idea how good those makes are. The Kia filter cost from memory £57! This came with an electrical connection and lead which I have no idea what it is for unless its a water sensor or heater. This attachment had to be bought because the original would not fit which I think is due to the fact that Kia have upgraded the filter for this model.
Anyway, hopefully I will post a "fix" in a couple of weeks.
Regards and thanks again.
Pete M