I see, some Sire's cost over £1k which was what I was thinking you were referring to with your £800 budget. I've played numerous US fender basses which were (to me) not as good as so say lesser MIM ones I've played in terms of build quality, playability or sound.
I think you get good ones and bad ones of any mass produced range, just throwing money to get a good bass won't always pay off (pardon the pun).
I look for general reviews from owners, if the concensus is good then it's probably a good start. This applies to any bass of any price bracket, then getting it set up well is very important, as are little tweaks like getting the fingerboard rolled - just makes playing a bass so much nicer, and often overlooked.