is it actually engaging 1st or reverse, then dropping out, or not selecting a gear at all?
presumably the prob resolved itself at some point as the garage couldn't find anything on the diag?
do you have the sele warning light on the dash?
you'd like to think that the alfa peeps would have performed basic checks like battery, selespeed oil level, relay and connections
, but my dealer wouldn't even look at the car for a similar problem (not even 4 years old), so i ended up getting educated on this forum and doing them myself - try searching for the above checks if you're not sure how to do them, or why.
if you do need a battery, i sourced mine at tayna.co.uk and got the identical one (manufacturer, and therefore dimensions and shape.) good price and service, too.
also, try the following link;
Alfa Romeo Selespeed - Home
for a great overview of the system and 'faults' and 'downgrading' tables which i found very helpful.
hopefully it's a glitch caused by a sick battery.
try starting a new thread, too. the tech guys will probably find you quicker that way. if you mention where you are they might point you in the direction of a trustworthy specialist who can help you...