never expect warranty from a mobile mechanic..
or backstreet garages.. anyway they do the work for few bucks..
if you want warranty both on parts and work done, you know where the services are ..
I'm also facing a similiar dilemma for some jobs here, even the service prices less than half there..
as we all know cambelt changes, engine rebuilds are risky jobs which can end up with another rebuild.. for that reason I prefer services..and here is my experience , the last time I changed the cambelt , the set was faulty..

which one mechanic friend (has a shop and can already change but prefer not to) and I opened it after suspicion and see the problem.. and after some debate with service, they accept and changed from warranty.. after the second set, some time later cambelt was again loose, again they solve the problem but this time dont change it (normal)
and after, again become loose
so even service warranty wont work properly as expected..