Hi m8
I can't speak for all the diesel additives only the Red X one.
Sorry to say that your fuel returns for a 1.6d drive e does not sound good.
If you access Parkers guide it will tell you you should be getting about 65mpg return on motorway driving.
The figures you are getting return are the same as my missus D5 which is a 2.4 ltr 185bhp and that is a 5 pot lump.
May I suggest you try what I said and use your cruise control aswell on motorway runs as that seems to be the magic button for economy.
With cruise on on motorway driving at 75mph ish gives revs @ About 2.225 ish (she has 5 speed gear tronic aswell)locked she gets about 47-48mpg and around 40-43 around town not at all bad for a 2.4 5 potter.
Also tyre pressures seems to make a difference...too soft and you will use more fuel and wear your tyres out quicker.
Check on your drivers door pillar on the label for economy pressures and stick to it.I think my missus runs on 36 psi and the car seems to like it.
Also pot holes are notorious for draining power (I know what your welsh roads are like and how the weather can suddenly change)so if you are about to change your tyres at all I recommend something like Good year Vector 4 seasons in an XLflavor for the few extra pennies involved because they have a reinforced side wall and keep a better shape when encountering crap roads or rain or heat or ice and snow.
Being 4 seasons tyres you don't have to change them summer and winter either just stick them on and leave them.
Www.camskill.co.uk
always have deals on and will deliver to your designated fitter who should charge you about £10.00 per corner all in(incl vat) to fit.
Even if you only change the front tyres being the driven wheels you may still find quite a difference on your fuel returns.
Hope this helps.