All S1 windscreen wash bottles are under the wing and only the filler-neck sits inside the bay...even on the non-UK cars. After all , they are the same car but with the battery & fuse box/ wiper motor the opposite way round. There's no room in the bay for the windscreen bottle due to the fuel filter and huge air filter box.
The early cars had room as the air filter sat on top of the engine.

I think you have an early car tbh mate and not a genuine S1 UNLESS the inner wing has been replaced and they haven't punched through the panel where it should sit and bought a 'bay tank' to fit instead. (?)
Get some pics up: visit www.photobucket.com and uplosd, then post the [img] link.