How can i remove limescale from the interior of an ALUMINIUM Burco Gas boiler?

There are products for this but in essence it is using an acid to dissolve the lime scale. Vinegar is the best one. If you could feed the vinegar into the boiler then let it work about 30 minutes, fire up the boiler then blow the water and steam out once you have pressure, it should do the job. It is always best to do 2 "acid washes" that are mild rather than one really strong one.