I remember when the SRN6 first went into service in the UK because there was a Blue Peter feature on it. Wow.
I also read about the later SRN4 which is much larger.
I flew/sailed on SRN6s numerous times, they were the staple craft on the Southsea-Ryde trip for many years. I've also been on the SRN2, 3 and 4.
And you're right about the 4s, they were HUGE. I even worked aboard one of them for a short while too.
There's one working SRN6 still left in the UK, it's the singular SRN6 Twin, based at the Hovercraft Museum at Lee-on-Solent at the last count.