Pressure loss in Ferroli Optima 901 when on timed programme?

On timed programe ie.off mid-day to 5p.m. and from 1a.m. to 7.a.m. there is loss of pressure below cut off point. Have had pressure relief valve replaced but no improvement According to Ferroli could be new pressure vessel needed or blocked pipe to latter. The former would require removal of the boiler And £40 cost of part .The latter is a pipe that is available without dismantling but Ferroli would'nt tell me how to identify this nor how to unblock it. The system has been treated with Sentinel 400. Is there some way of repressurising the expansion vessel? Or getting a more accurate diagnosis