The oil filter and slave cylinder repair kit are the same part on both 1300 and 1500 Dolomites. However there ARE some aftermarket slave cylinders out there that won't accept the OE seal kit and your car may have been retrofitted with one of these - so it's not always simple!
Ah, that does complicate things as I have a receipt for a slave cylinder from a couple of years ago. Is there anyway to tell the originals from aftermarket?
If it's only a couple of years old and it's failed already, it may well be a cheap aftermarket one. IIRC Jonners (Jon Tilson) had a couple of OE ones that he had carefully reconned not long ago. These slave cylinders are a bit of a bugbear for members, not as potentially destructive as the poor Sprint timing chain tensioners that are around, but a PITA nonetheless.