I vaguely recall Ricta Cores around when I started skating (tail end of 08 I think) and everyone who had them had issues with the core popping out - a friend of mine randomly stumbled across some "all star chrome cores" with gold cores and purple urethane in a junk shop in the middle of nowhere. Infact my first set of "proper" wheels were Ricta Supercrystals in a colour I can only describe as 10 beers deep dehydrated piss.
I haven't skated the Sparx, but given their description and price point at £25 I'd assume they're just a generic price point wheel up there with Bones 100's etc. That being said what's the prices of Ricta Naturals over there? They're often on sale here for equivalent or cheaper than the Sparx (£20) and in my opinion I'd say they're definitely comparable to F4's, STF's etc in terms of the slide. They definitely do cone out and shrink a little quicker, but overall they're a nice wheel and I'd have no qualms skating them again.