I stripped a little more paint this morning:
It looks very much like the original paint was an orange-yellow-blue fade.
Another search online and I found ...
This photo was labelled as an Ozark and originally an Ebay listing, for which the original photos are unfortunately no longer available. This is strong evidence it was probably originally an Ozark. If anyone can find some better quality photos of this paint scheme Ozatk I would be grateful!
The remaining discrepancies?
* The orange in that picture looks quite "tangerine" but the orange on mine looks very fluorescent (especially inside the BB shell). This could easily be due to the change over time of the Ebay one and representation of colour in the photo not being 100% (though fluoro usually fades rather than deepens).
* My frame looks like it faded to blue, rather than white. This could have been part of a later colour scheme, especially if they kept some of the original paint after the modification and just reapinted the top section. Poosibly could also be cause by some breakdown of white paint?
* The fork is different. Mine has Gipiemme dropouts and no rack mount. I've stripped enough paint from the fork to see it has the same original paint scheme, suggesting it was original to the frame. I guess this would just be due to some variation in what forks Raleigh had available.
* No rear rack mounts - I assume these were removed when the frame was modified. Again, there could also have been some variation - the 1989 catalogue shows rack mounts on the Avanti, but examples do not all have them (see blow).
* The paint in the 1989 catalogue is the more common red-white fade. I haven't found this one in a catalogue.
Additional supporting evidence
* The Avanti was full 531 ATB but the Ozark was 531 main tubes. I think the seat stays on mine look more like the Ozark above than the more slender ones found on the Avanti:
(note no rack mounts)
* The headset fitted when it came into my ownership is a Tange Unique "Loose Proof" ... which is listed as the spec for the Ozark in the catalogue (Avanti XT II and Appalachian Deore II)
Overall, fairly confident it was an Ozark that was then modified.
Next step, get a couple of seatposts from storage to check whether it is 27.0 mm or 27.2 mm, then build it up and see whether there's any way of creating a ride position that will work for me or whether it is insurmountably small.