Back to the mini filter, needn't be a bear of a job, just need the right tools and it's an easy one.
Get a reasonable socket extension and an extending magnetic pick up thingy - Tesco were selling them for a quid the other week, will get he banjo bolt out and more importantly, back in again without needing to remove anything else.
Once you have the bolt out, you need to go filter diving, mine was a surprising way down in the pump, but others have found theirs inside the bolt itself. I used the metal reinforcing from an old wiper blade with a small hook made at the end to catch the filter. Opinion is divided on what to do with the filter, either bin it or clean it. I dropped mine into the dark recesses and never found it again, so guess which way I went!
My TD was suffering exactly the same problems, really starting to hold back, misfire, etc. etc., one wee filter removed later and easily better than it had ever been for me!