1) Is it reasonable that I can claim my lost earnings back as it was as a direct result of not being sure if the car was roadworthy or not and dealing with registering a complaint that led to me being late and losing money?
If the car was unroadworthy and as a result there were losses then that is more likely to be compensated. But just a possibility it may have been and trying to work out if it was is unlikely to be compensated. Still, nothing stopping you from trying your luck and including it in a claim.
2) Is it ultimately the car wash's decision or my decision on where to take the car to be fixed?
Ideally a mutual decision, but ultimately there is an expectation to keep costs down as long as it is done at a place which is capable of doing it to a reasonable standard.
3) I've put the wrong date on the letter (a month out). It was a simple typo when I was writing it, but will that cause any issues when I go to small claims court or will it be accepted as a typo and not cause a major problem?
Not an issue as long as you identify it as an error
4) What is the correct way to put in statements from mechanics A & C? Can I get them to email me a statement each or will they need to come in in person for them to give evidence?
They can give a statement you can include as evidence but they should be available to attend if needed to be cross examined otherwise their evidence can only have limited effect.
5) Is there anything specific I should know about claiming this sort of case in small claims in order to improve my chances of getting a successful verdict?
It is for you to prove that damage was caused by them so try to get whatever evidence you can to back that up. The rest is really down to the judge
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