i'm trying to think of a middle ground here. you really need some sort of proof that they've carried out this work.
what i would do then is:
call them. tell them you do have a slight concern over the crack on the sump and you're just looking for reassurance that the repair they carried out is sufficient and will not deteriorate any quicker than an equivalent uncracked sump would. when they say yes it's fine, then great, we move onto the next step. they won't say no.
send a letter. a nice one. detailing every point between when you bought it and now. sort of like "thank you for swift action with regards to the rectification work on the above mentioned vehicle." (you'll refer to renault clio 172 reg no in the subject of the letter).
"having bought the car on such and such a date i brought to your attention on date the requirement for some immediate rectification work due to an oil leak. the rectification work carried out was to replace the cam seals and this did indeed resolve the issue of the oil leak. having brought the car home on date i noticed there was still evidence of an oil leak and so returned to you on date. this was quickly diagnosed by yourselves as a hairline crack on the oil sump and was repaired with the use of "liquid metal". having spoken to fellow car enthusiasts it was brought to my attention that this type of repair may not be as desirable as the fitment of a new oil sump. on date i called and questioned the longevity of such a repair and you have reassured me that there is no cause for concern on the repair of the oil sump. upon your reassurance i am happy to continue using the car as intended and i look forward to many years of enjoyable motoring. i would be grateful if you could, by return of post, confirm this summary of events to be accurate."
some sort of bolx like that. send it recorded signed for. basically be as nice as pie and list as much info as you can. they won't respond to this letter which is fine.
but if in three months time the sump goes and you're faced with a repair bill for a new engine, you can use it as evidence to help recover costs from the garage. they won't have written back initially, so they haven't disputed your original version of events. if they do write back in the first instance, then it's hello mr new sump.