[WillPS]

www.itvplc.com  might be of interest, as might itv-vs-hub.kustomer.help 

Alternatively there are various episodes on YouTube, nowhere near complete of course and in varying types of quality - from potatovision to semi-decent.

Might be able to obtain what the BBC call a contributor copy (don't know what ITV call it), but may have to pay for it.
Should also be noted that if we're talking about Series1 of Blockbusters a few episodes are missing.

ITV Viewer Services will be no help, it'd be the ITV Archive (www.itvarchive.com ) who might be able to help but they state:

Registrations are limited to professional and commercial users only. We’re sorry but it is not possible to grant access to members of the public.
New clients, you will need to register first in order to licence content. Once registered you can search, view, request information and order content using the ‘Info Request’ screen.

No harm asking them ( clipsales@itv.com ) - give as much info as possible (bear in mind that it's unlikely they'll have a record of individual contestant names and be able to retrieve that) and be prepared for the inevitable response or no response at all.

At the risk of sidestepping the OP's question - is there a significant other of this retiree you could reach out to and see if they have a copy? Even if it's a video tape it'd be way more straightforward than trawling through the web to find this.