In fact the power is more 130ps than 115ps.

How do you know this? Has it been chipped? A simple tuning box cause over-fuelling and black smoke.

Come MOT time I put £5 of petrol into the tank

Yes, petrol will get rid of smoke, but it'll do the HP pump no good at all. The pump could be struggling to build sufficient pressure to atomise the fuel.

Fuelling should be dependant on boost ie the ECU should progressively increase fueling as the turbo spools. Old mechanical diesels used pneumatic boost compensators to adjust the fuelling to the level of boost in the manifold. Your TDCI will have an electronic equivalent. Again, a chipping device might override this.

Other than that, the MAF may be dirty and giving incorrect readings of air flow to the ECU-although this tends to cause under fuelling and sluggish performance rather than black smoke.

The TDCI engines in the mondeos are quite well known for this type of smoking. We used to have a 2004 mondeo TDCi which did exactly the same thing. When we asked the technician at the ford garage he said that these engines quite often do this. If the DPF is clogged, it may help a little if it is replaced/cleaned etc. TBH though, there is nothing wrong with the car as a lot of these mondeos i have seen also do this :)