Oops, was typing, missed your post. Yes, the ex-Vaillant guy who saw it last year said as much about the boiler, he was quite passionate about it and the most fair and professional person I've ever had found in any trade. You can find him of you search for Vaillant service London.
Not much room for an external EV, works have to be mounted behind a kitchen cupboard or in the underside of a worktop if it has to be near the boiler. Hopefully it might be a pump up job.
OK, as Footprints has said, depressurise the ch system/boiler, then try pumping up the EV to .75 Bar. Leave for an hour or overnight, and re-check EV pressure. If still .75 then EV will be ok. If it's ok, repressurise ch system/boiler to 1.2 Bar. If however the EV loses pressure in the test period, it's time for a new one.
And if you're using Mark from Vaillant Service London, you are lucky to be using some of the most knowledgeable people in the country when it comes to Vaillants. A Turbomax Plus maintained by V S L is a great combination and should ensure a few more years of economic and reliable service of your heating and hw.