I've googled lots and struggled to find anything compatible.
Even spoken to land rover and ask they can recommend is a tow bar one that is much more expensive than I need/want, hence trying to see what others have done...
What is it about after market carriers you are not finding compatible? I had issues initially with finding a long enough reach on the bike rack mount. Rover hitches are tucked back further to ensure a decent departure angle. Hitch carriers are more expensive due the heavy build need to hang bikes in place safely if using frame mount carriers. You can get a tray style carrier that use wheel mount clamps and stabilizers but they are still pricey for good quality. I opted to buying a heavy, quality carrier instead of risking $4K in our two mountain bike breaking free.
I would recommend sticking with a hitch system as the hooks, straps and pads for tailgate carriers can cause damages.
It's mainly for carrying my tot kids bikes, hence I don't want to spend hundreds/thousands on a tow bar system, just something that works for occasional bike carrying - but bike shops keep telling me nothing will fit the boot door as the bottom of the door is too thick. I can't believe no-one else has had this problem or that no manufacturer has a solution! Someone must make something that works?
if they are little kids bikes toss them in the back with the front tyres removed. Obviously you have no roof rack or you would be looking at those options as well.
As far as your last bits:
No, most people do not have your issue because folks that carry adult bikes around use a hitch mount or roof mount system. They require their bikes to be secure and safe.
Yes, manufacturers have other solutions including everything from suction cups to straps and pads. None of them cost thousands of dollars. :roll: As stated above use Google. You failed to answer what is not compatible about everything you have found, so there really is not much anyone can add here.
i'm not sure you're gonna find anything that meets both your requirements for 1) fitting kids bikes and 2) not spending hundreds. i've used two different hitch mounted racks thule and currently using a softride. when the kids had small bikes, i just threw them in the back like RRToadHall suggests. when i was looking, most seemed to be for adult sized bikes and you needed an adapter to fit kids bikes (more money). those that didn't were the tray style carriers also carrying a higher cost.
Back in O5, I found a cheap one at Walmart that fitted over the back of the hatch of my LR3; set up easy and very adjustable (later used it with the Jaguar and now fits my Lincoln), impedes the use of the rear wiper, however. Just found the Thule with the LR receiver much cleaner and used it for years. But not really cheap (~$200 for the receiver, and ~$190 USD for the Thule -- sorry for not knowing the exact prices, long time ago).
I am willing to bet you can find a LR receiver (LR3 or RRS) in one of your local breaker yards; I see used Thule racks in many thrift stores for very little. When my grandsons come to town, however, I tossed their small bikes in back.