Thanks for all the advice - going to check out the levels shortly and see what that is. It does seem to drive perfectly fine, I've owned 2 zeds in the past, and though I'm no mechanic, it seems to drive perfectly well, no rattling, knocks etc. and engine sounds perfectly smooth, so hoping it's something minor.
Minor complication is that the person I bought it from put the advert on autotrader as a trade advert, specifically told me in a text it's a trade sale, so I have that there, but the invoice simple says his name and doesn't state a company name, albeit he told me he was an independent dealer, so not sure if it would have a company name on the invoice anyway. He also put down 'sold as seen' which I know people do in private sales to cover themselves but apparently trade sales aren't allowed to include that anyway, as per trading standards info - didn't know this until I checked last night!
I'll check the levels shortly and sort that out. I was also gonna check with the local garage, see what they say and if it's something major then I'll be going back and asking for my money back and if I have to go down small claims process for it then so be it....