How to fix flashing on leaky roof?

I live in a rural area with few if any reliable roofers--Have small leak in my tile roof (at a vertical pipe) and roofer wants $2500.00 to fix it--he's the only roofer/repairman who returned my calls. So I need to try and do it myslelf. Roof was built 20 years ago and instead of using flashing they just surrounded the pipe with a big glob of cement which is now cracked. How best to fix? Seems I'd need to remove some tiles to install the flashing but how can you remove just a few tiles and put them back when they are all laced together-(These are Spanish tiles shaped like a flattened "S") Thx.