Did you make the Young's up with sugar? If so, this may be the reason it tastes 'tarty'. If that is the problem, you may find that this taste will diminish as it ages. If your lager is flat, it would indicate that the priming sugar hasn't been used up yet, so it should really taste a little sweet if anything... so perhaps something else went wrong...
As steve says, you need to keep the bottles at fermenting temperature for a week or so to condition. Although that will build up a good bit of pressure, I always find that lagers take months to reach the level of carbonation which is necessary - for some reason it builds up much more slowly than in bitters, etc.
Even when using an ale yeast, I've made some reasonable approximations of a lager. However, to get that lager bite in the taste, I haven't found any alternative to adding extra hops - there's been a few posts in the kits section discussing adding hops to lagers.