Are you saying, lSJMS, that a Krups machine is to be preferred to an Oscar?

Of course not. I'm saying awesome, fresh coffee from a good roaster and poor machine will make better coffee than a great machine using stale, old supermarket coffee.
When I first got a HX it took a bit of reading to get my head round the operation and flushing. Read up on the "water dance."
For me UK skimmed is no good, hard to foam and thin as you say. I use full fat, but have small capps, semi is good as well. Try a smaller but richer and tastier milk drink, same calories but double the flavour. Cravendale was used by many in the UKBC, full fat though.
So get good beans(Jman's list is spot on), work out how to hit your target temp on a regular basis (think hot, med, cool not 92.5etc.) and perhaps do that stepless mod on the Rocky and I think you will be pleasantly surprised with where your espresso can go.

Isjms, all your points noted with thanks. Just returned from time away. Have looked briefly at info. on water dance and will continue - haven't yet fully determined at which point the dance stops and have been flushing mainly until the steam from the issuing water ceases and then giving it a further couple of seconds. Have been experimenting with different milks and might find that a mix of a fatty milk with some Cravendale skimmed would ease the conscience whilst making it more flavoursome than all skimmed. Have, in the past, fiddled with making the Rocky stepless and would reconsider it, more permanently, after getting better beans.

Is there much debate regarding when to start timing a shot, ie. at the very start having turned the pump on versus waiting for the flow to start.....there being about 4 or 5 seconds difference?

You should be able to see when the water stops boiling by removing the PF with the pump running and watching the water. It bubbles and hisses when it's boiling. Once the flow calms down and starts running smoothly, start counting. The longer you count, the cooler the water will be.

Time the shot from when you start the pump, unless your machine has built in preinfusion, in which case you need to subtract the preinfusion time from the total shot time.

"Time the shot from when you start the pump, unless your machine has built in preinfusion, in which case you need to subtract the preinfusion time from the total shot time".

Thanks.......but I've read here that there's further debate as to whether Osacar actually has pre-infusion. Some have said it has, other's not and yet others say that I can partly push in the pump button for a few seconds in order to get some flow, without the pump operating (which, indeed does seem possible). The advertising says it has and I do believe there's a delay, at the start of the shot, where the flow is more gentle followed by normal delivery.

I think all machines have some sort of pre infusion as the pressure builds in the pf. As the owner of a manual lever, I say forget messing with on the fly pre infusion and master your machines basic operation. This will reward you far more than adding another variable.
I follow a ruthless KISS methodology so any use of timers is not for me. I always end a shot by eye and after a short while you can tell the time from the fall of the "Mouse tails" but a 25 count from the pump turning on gets you in the Golden ballpark.

I know it's hard but try not to mix it up too much. It makes it hard to steadily improve your espresso and you get into these wild peaks and troughs. Try to let the machine do it's thing rather than trying to manipulate it, work on your dosing, grind and distribution, this is the crux of espresso(imo) Just try semi only for a while when you have nailed the semi then change. Same with the flush, Beezer, said to count from when the flow calms- good advice.