My rear derailleur needs constant minor adjusting all the time
every couple rides or so the chain will skip slightly or sound off and require a 1/4 or 1/2 turn on the cable adjustment which I do on the adjuster on the downtube whilst riding.
it is becoming quite annoying, I have checked that the cable is not sticky etc.
I wonder if the derailleur spring is losing tension.

Any suggestions ?

Yup. Check out your cable housing ends.

If you can see one or two little wires sticking out of the cable housing end - that's it. Your cable housing is gradually shortening itself. Recut the cable housing nice and square, install a new cable housing end and you'll be good-to-go.

I doubt the spring is losing its power/resistance.

I would check to make sure that the cable is not slipping from its anchor point on the paralellogram. I would look at the cable and tighten the bolt/screw/(choose your fastenter) that keeps the shifter cable in place on the deraileur.

A big clue to the nature of your problem is whether you're making trim adjustments back and forth, or whether it's always in the same direction.

For example, if you're always tightening the cable it's probably slipping, or the housing is extruding through a ferrule as the prior posters suggest.

If you're adjusting back and forth, it's neither of the above, and probably sticking slightly within the housing or the lever mechanism isn't as crisp as it should be.

Next time it's out of trim, don't try to fine tune it. Instead pluck the bare wire away from the downtube like a guitar string. If that works, it's definitely a sticky problem in the cable or lever.

In addition to the other suggestions, make sure the bolt that mounts the derailleur to the frame is tight. If your frame has a removable derailleur hanger, make sure that bolt is tight. Make sure the derailleur's barrel adjuster is securely engaging the threads of the derailleur.

Sure would help if you'd tell us what kind of components you have.
Could be a bent dérailleur hanger.
How old are the cables and housings?

If you can see that it means you've been using the wrong ferrules to terminate your shift housing. Not that you should recut it.

I'm thinking this is something that you've never seen. If you had you'd realize the necessity of squaring off the damaged cable housing end.

Not true; when replacing shifter cablesets, they come with the proper ferrules, and they can fail just as described. My last couple sets have done so -- and no, it's not 'operator headspace', I do that kinda stuff for a living.

Poor Operator.

He really does know his stuff but he's so anxious to show he's the smartest person in the room that he'll dig through a whole mountain of gold to find one little morsel of pyrite.

Another possibility: the derailer is so old that there is play in the pivots.

its Ultegra 6600 shifter and rear derailleur
I loosened the cable adjusters on the downtube by the headset and at the derailleur and readjusted everything.

What I realized is that the plastic ferrule inside the turn adjuster at the head tube binds a little with the turn adjuster
and when the handlebar is turned left or right it tends to cause the adjuster to turn a bit with the cable flex which appears to be the problem causing minor cable adjustment requirements.
The cable housing and ferrules are shimano and are in good condition.

What can I do about this ferrule which is binding with the turn adjuster ?

If the ferrules are correct you can lube the ferrule/adjusting barrel area with grease (or a lube of your choice). You may look for another type of cable stop/adjusting barrel. Your cable housing may be cut too short. Do you have a picture of the bike that you can show us? I would like to see something that would include the adjusting barrel and cable routing to the STI levers.

Agreed. If turning the handlebars is causing the adjuster to turn, the housing is too short.

I have that adjuster set to minimum so less tension on that cable segment for hopefully less binding there
I don't think the housing is too short there

Nice looking bike - cable housing looks good. Housing does look long enough. Have you checked to see if the cable is fraying inside the STI lever?