I am going to respond to this section by section.

Hi Everyone

I have the metabolic rate of a sloth, always have had, and as I get older (since hitting the 30's especially) it seems to get worse all of the time.

I know that increased muscle mass speeds up the metabolism, and diet is the key to losing body fat, eating small and often etc.

True

Basically I have tried everything known to man and beast. My problem seems to be that as soon as my body gets used to a diet/eating plan, it gets complacent and stops losing fat and starts piling weight back on.

Something is off here, without more info its hard to say other than I suspect a significant loss of lean mass over time causing this effect.

Example, I have been doing Dr Di Mauro Pasquale's Anabolic Diet now for around 12 weeks at least. Those of you on leangirl will know I got fantastic results in the first 6 weeks, losing 10lbs, and I am certain that the lean mass lost was a bare minimum.

Just for the record, those are not "fantastic" results. I would be pissed if a client lost that little fat over 6 weeks and would know they are cheating. I have had women lose as much as 30lbs of FAT in 6 weeks while building muscle. I know I am a guy but when I did that diet 10 years ago I lost 12lbs of fat and gained 8lbs of lean mass in 4 weeks. So something is missing here.

Then after sticking to it religiously for a further 6 weeks, my weight has crept back up to 2lbs less than when I started.

THis shouldnt be the case, you probably are eating foods that put you over the carb limit for the day unknowingly or your calories are not correct.

Admittidly, I have changed my training, lowering my reps to 5,rather than 8-12 to build up some strength, but I know that the gain is not attributable to muscle mass!

Correct, so stop doing it. Are you trying to get strong or maintain and or increase your lean mass? You are admitting to doing something that you know doesnt make sense for your goals.

I never used to do much CV, but have now felt the need to buy a life fitness stepper at home and i'm doing 20 mins HIIT every morning before breakfast to try and speed my metabolism up that way......ITS A LAST RESORT!!!

CV doesnt increase your metabolism, do it in the AM to burn some cals when carb depleted but YOU KNOW only building muscle with give you a long term increase in your metabolism.

Does anyone else experience this, in that they get used to something so quickly and need to keep on swapping/changing....ITS SOOOO FRUSTRATING

Its hard to say why it isnt working for you but clearly it isnt about too. If something isnt working its time to change. POst your present diet (and be specific) on the "critique your diet" thread and I will see if anything jumps out.

Darrin