Slow progress so far. UK sill covers I ordered on eBay were non-OEM and are thin grey smooth plastic type that need priming then top/final coat - my fault, I should have checked first. Taking the hit and return posting these.
The LLL part recommended earlier, when I checked availability, replied back to say part # 1K4 853 859 B 9B9 is no longer supported/offered, which is weird as it looks the very part in their eBay image. Started to make me think that there might be a difference in length between my 3-door vs 5-door. Can't imagine VW would manufacture a different shaped trim/cover part just to cover the same sill? Maybe I'm wrong unless anyone can confirm. Found instead another eBay site (lithuania) which said they did have that part number. Now clarifying with them it's a left-hand fitting for a 3-door GTi, and not something else.