the filling loop is the chrome flexible hose under the boiler not sure if the alpha has built in filling loop if so it is located on the under side on boiler on one or both ends of chrome flaxible hose there will be black lever valves,or white valve heads that you can turn open these up and the presure on the boiler will increase when you get to 1.5 bar{when system cold ie no radiators on} turn valves off
if you do not see and valve heads you may have the slot type that you require a screwdriver for if this is the case look at each end of the chrome flexible hose and you will see a fitting at each end one of these fittings will have a slot in that in accrss the valve place screwdriver in slot and turn slowly max quarter of a turn and you will hear water flowing into boiler once again when 1.5 turn off
you may find when you turn off presure will drop slightly again if so just top up again
tony...tsp heating, July 2007