Re: Any ideas why?
When you say "picked up" I presume you mean bought, not that you took it in for repair and that is what had been done to it. I also presume that you are a private customer and not a dealer in traded in cars buying them from main dealers.
You have the right to reject the goods (the car) under the Trades Description act if the goods have not been properly described, that means that the dealer must describe the car properly both in the way he applies the description and the way he does not, in other words the T.D. act covers acts both of commission and ommission. A simple example of ommission would be an antique dealer who purchased (cheaply) an item from a member of the public without telling them the real value and then went on to re-sell it for a massive profit.
However, you too have a duty to ensure that you have properly checked the goods before purchase, they have a defence of stating that you were allowed to inspect the vehicle and reject it before purchase. Normally you would be able to counter this type of defence by claiming that a "reasonable" person could not have been expected to notice the fault (eg if the incorrect brake fluid was in the reservoir but the brakes appeared to work, as a reasonable person you would not be expected to know that to be the case as you would need to have the fluid chemically analysed). Your problem here seems to be that the "faults" were immediately visible as soon as you opened the bonnet. Why didn't you look under the bonnet before you bought the car? That's the way their lawyer will put it.
The sale of goods act gives you further rights, goods must be of a merchantable quality and give reasonable length of service, dealers can no longer hide behind a "sold as seen" escape clause but are required to warrant the goods they sell for a period of 6 months.
You should return to the dealer as soon as possible and ask them about the problem, maybe they didn't know about it (1st excuse), maybe they thought it didn't make a difference to the condition (2nd excuse), maybe they thought you should have looked at it properly and assumed that you accepted it (shifts the blame back to you). If you are a member of the AA/RAC contact them for an inspection and advice. If not contact your local trading standards department.
If you purchased the car using a hire purchase agreement or credit card then the finance company is responsible and you have a chance to have the agreement revoked and the payment refunded.
Remember that the law requires you to act reasonably, you can't sue them if you haven't told them about your complaint, so your first action should be to get it independently checked, then return to the dealer and complain in writing. Give them the opportunity to rectify the problem within a reasonable time. If their response is negative then proceed as advised by the AA etc.
Also remember to add a heading to all your correspondence with them E&OE (means errors and ommissions excepted) and to accept in writing any "repairs" or modifications, substitutions "without prejudice" so that you retain your rights to reject the car if it is still faulty.