Leaking roof - advice needed please

[ptown]

Hi
A couple of summers ago I had a lot of work done to my main roof (all laths replaced, soffits/fascias/guttering replaced, broken tiles replaced) as a result of an investigation into a very minor leak. It turns out that I managed to hire a cowboy to do the job (plus other work).
That builder is now uncontactable, and I went into the loft today and saw a very minor leak (it is raining quite heavily) - just a few drops, but one of the laths is quite soaked, and I realise I need to do some remedial work pretty sharpish.
I've asked a few friends, but nobody has had any work done to their roof, so don't really have any recommendations. Obviously (due to past experience) I am now very apprehensive, but I've seen a couple of local roofers in the little advert booklet that comes through the mail.
What questions do I need to ask to ensure I'm getting a reliable roofer? Are there any qualifications they need to have, or do they need to be part of a special organisation/institute/assocation that guarantees reliability/integrity/etc....
Any help is much appreciated. I'll try to get some photos up too.
Thanks

i've been a roofer for thirty years , first cement mix is as said , not a strong mix , its three parts sand one part cement as spec from redland roof company as for the rest , god i feel for you , word of mouth is the best recommendation my friend