I've name-changed for this as have also posted on a Facebook group.
I took my clio into Renault last month as there was a knocking sound coming from engine. They quoted me £1457 including parts and labour to fix/replace the broken bits.
Car is nearly 10 years old with high mileage and therefore £1457 is pretty much its value. BIL's friend owns a garage and said he'd have a look.
BIL's friend quoted me £750 including parts and labour as he felt one of the parts Renault was going to replace he could 'recondition'. He said it'd take a week tops.
After a week I called and he said it's probably be another week as he was having trouble sourcing a part. Fine 
. End of second week and the part is apparently not an item held on the shelf but it'd be ready today.
I went to garage to pick car up today and it's not fixed- driveable to home but BIL's mate says it needs a new engine- basically the £1457 Renault quoted me. I've got a bill for £700 (he knocked £50 off for me as a favour 🤬) but still a broken car albeit one with a few new bits that Renault will likely have to take off to fit a new engine!
I've not paid the invoice yet. Can I withhold it all some as he's not actually fixed the problem which is what I wanted!? It's actually cost me more than £700 as have had to pay £300 in train fares to get to work etc compared to about £60 in petrol.
I'm worried he'll take me to court and I don't want a CCJ watched too much Can't Pay We'll Take it Away...