Hi peeps and thank you very much for your input. Firstly apologies for being "offline" for the last couple of days. I'm on some medication at present pending a visit to the Q.E. hospital in Birmingham in a few weeks time. The dosage tends to knock my sleep patterns and can leave me a total drongo for periods.
First off I'm pretty sure that the Outback inverter charger does nothing with frequency shifting; simply responding to time and amps in/out setting parameters.
Secondly, HH you could well be right regards the immersion heater thermostat being the actual cause of my problem. Possibly "fluttering on and off as it approaches top of it's heat setting.
I spent some time today doing an up to date drawing of the circuits as they stand today. I have missed out detail of fuses and some circuit breakers for clarity but here it is.

Rather than spending too much time trying to discover the why of my problem I have formulated a plan to try and I will do a drawing of the planned amendments to post tomorrow hopefully.
I rediscovered a Coleman Air diversion charge controller in my box of tricks and propose to try/test using it to control the AC dump load, rather than the PWM signal created by the Outback FM80 charge controller and SS Relay which is clearly the source of the "disconnect" problem for the GTI.
http://manuals.colemanair.us/shared/DiversionController160M_Current.PDF The Coleman is designed to dump a DC load, but I propose to try to use the control circuit to dump AC from the house grid to my AC dump loads. I plan to try connecting the AC in from the WT to the common of the relay and then on to the grid. the connection to the dump load will be on the NO connection. When the relay switches the WT will still see the grid but will be able to 100% dump directly to the divert load. So hopefully avoiding and frequency issues. The test will cost me nothing as I've got the kit in stock and will require minimum disruption.
Basically the WT input is sent on to the dump load at an adjustable battery volt setting for 5 seconds. The Coleman then rechecks the battery voltage and either repeats the "dump operation" if battery voltage is above the set point or not, if not. As supplied it has a 40amp DC relay which I will replace with an AC jobby.
For test purposes I will disable the FM80CC aux signal so that the solar controller will simply complete the battery charging with no attempt to divert additional solar harvested power to the dump load via the AC bus. If ultimately that means that I can't get a free tank full of hot water from the PV in the summer months (all sun and no wind) then I'll settle for that, if it means that in winter the WT operates safely and at maximum efficiency.
With regards the 2nd issue with the genny coming on and "upsetting" the WT supply and disconnecting the Aurora GTI, 1stly I will reduce the genny run time to either finish at a lower set voltage point or a limited set run time 3 hrs perhaps.
At present the genny MUST run if battery voltage drops to a set point and then completes a full charge cycle to the absorb voltage. In reality usually the battery is also receiving some charge from a RE source and to some extent the top of the charge cycle is unnecessary(ISH). The genny will also autostart if the load is in excess of a set point but then only runs long enough to support the load demand and then "off's" itself. In the event of a large load required I usually manually start the genny so am on hand to supervise.
From my records I see that the WT is providing in excess of 600W when the wind speed is 6 to 7 Mtr per second and I can safely use the brake switch at that wind speed. If the WT is supplying 600W then the genny won't get a start signal (normally) as that rate is greater than the normal house load. So I will fit a relay activated brake switch to the wild ac in from the WT to actuate when the genny starts. IE if WT is supplying 600W then genny won't be required anyway. But if there is no wind and sun and the battery is fully discharged then the genny can start and at the same time the WT will be braked and will remain so until the genny stops.
I'm sure the purists will disagree but I don't get hung up about the actual state of charge of the battery. It's never static and is constantly charging and recharging which is what it is designed to do. If you've seen how fork lift trucks get hammered day after day; well eh by gum. I got many years life from my 1st set and still got £400 for scrap after 12 years; which was a quarter of the cost of their replacement.
I shall have to purchase some relays to complete the brake switch plan as outlined and will report back on the modified divert load scenario. I will of course need some wind unlike today.
Thanks all for input and thoughts.
Andy