Are we then agreed it is illegal to make an instrument approach to 900 feet without ATC?
That is not 100% true.
If there is a published procedure then there must be ATC.
Some commercial operators have procedures approved by the CAA and included in their ops manual. Examples being operations to some of the Highlands and Islands airports or for example Blackbushe where companies have had procedures approved just for their own use and with only AFIS service.
However, those procedures will be the exact same as the published ones i.e. will have had the exact same design, approval and checking process as published procedures.....an expensive thing.
Home made GPS (or any other) instrument procedures will not meet the same standard and thus among other things could be classed as reckless endangerment.
A mis-conception among pilots is that if for example there is a VOR on or near the field then they can construct an instrument approach procedure based on it. However, the instrument approach procedure relies not simply on good professional survey and design but also on having the approach aid certified as fit for more than enroute use plus flight checked. The AIP navaid listing shows what each VOR navaid is certified to be used for. Enroute means enroute and no more.
Regards,
DFC