right... i went back in and got served by a different person this time.... he said they only had one size so i went with it (ut was the slightly shorter one).
When comparing them both the only difference i can see is that the lower part of the filter (which has a smaller diameter) is shorter, i can only guess that this section is mainly used as a sort of resivoir to collect any water that is accumulating? or for fuel, i.e. its got fuel waiting around at the bottom to be drawn through to the injectors, i.e so its quicker than having to pass it through the filter, so effectively it has a reserve of filtered fuel to draw from...... just some theories......
i wonder if the different filters have different effects on fuel consumption/performance......
since mine has been on i think fuel economy has dropped by about 2mpg, per the computer its lower?