Hello all, boiler not working properly?

Hello all, i have a very old combi c.h boiler (solaire model ghb 21/f t.02) which no one has heard of, i would guess at least 15 if not 20 yrs old. two probs with it heating seems fine but losing pressure ( will run ok on with zero pressure, which i'm sure is not right) bleed rads and top up weekly, have located small leak to diverter valve (prob 2), hot water tempremantal often running lukewarm , can coax hot water by turning on and off a few times and running water slowly. I would like to change the diverter valve (diaphram switching ok) to get it running whilst saving for replacement. problem is i do not know which one to get, .have found one in b.e.s catalogue (page 118 in the pic it looks identical to dvo-d) but there are 4 diff dvo-d types . only markings on mine are H70787 any help appreciated.