I've got a very similar situation with mine (just bought last Friday). From what I can gather there is no Haynes manual, nor any other for that matter. I believe the only options are either to create an account with Seat's online service system erWIN Online, and you pay to view per section as required. There are options to print but it can only be viewed on one device at a time. Very awkward altogether.
Alternatively, there are disk sets available on eBay which cover a range of models and they range in price from €14 - €35. At the moment I'm deciding whether to buy from a seller in Italy. Search eBay for "Elsawin *4.1* Seat 2013 - MULTILANGUAGES!!!" (I can't recall if us newbies are allowed post links yet, but you'll find those two options easily enough).
I have a temp sensor on order; a service oil message on start up, that clears after a minute or two; and an EPC message that does the same. I'm working through those at the moment.
Your service message could be the same as mine, and I'm guessing that engine management is indicating an oil change is due. I'm about to do that this afternoon as soon as I get hold of a replacement sump plug gasket/washer.
I don't know if that message will clear itself afterwards or if I'll need to get it cleared with a VAGCOM/VCDS or similar plug in
diagnostic tool. I have seen a video on youtube (watch?v=D1PGvVG8xaM) that shows a method of clearing a service message on other models of Seat, so there may be a way for the Arosa.
I'm new to them and still learning. If I find out anything else I'll pass it on. Or maybe someone here can point us both in the right direction...