I agree guia is the best place for this national dish, go to the local portuguese restaurants they have the best taste and prices 5 euros per person, try 'o grade and 'o aurelio.go in august and look out for 'festivo de frango' this is located at guia football club training ground in the town centre, approximately 100 chefs ply their trade here and for a nominal entry fee you can eat as much as you like
better still why not make your own piri piri it is much more fun, most recipes are a closely garded secret but I will share mine with you
from the market buy some "malaguettas" these are small fiery red chillies
or you can buy them in packet form already dried.
from fresh dry them in the sun, then grind them until crushed add them to bottle of the best virgin olive oil ,add portuguese paprika and last a slurp or two of decent port. shake and allow to to ferment for about a week then it is ready to use , excellent in soups or stews as well as a meat marinade.if you like it really hot just use more of the said ingedients.
bon appetit