thanks for all replies..

my guess was that at the cabs price point, there probably wouldn't be any elaborate sealing gaskets
or any thing else fancy construction wise.
But I did suspect the possibility of at worse glue seeping where it shouldn't if the cab had been assembled
too hastily in the factory.

So after a couple of hours preparatory googling I found one guitarist forum thread with the encouraging entry

"I even took the back panel off (it's very tight, so be careful how you pry it off)"

Anyway I was up carefully trying to ease the back off
with expired credit and video club membership cards
until the wife's alarm went off for her to get up for work..

and still I hadn't made any progress..

So when I woke up this lunch time I thought eff it, I'm taking a kitchen knife to the stubborn bastard..
Ater an hour or so I finally eased the back off,
fortunately with minimal abrasion to the edges of the vinyl surfacing.

Ok it's not a hand built British marine grade ply boutique cab,
but for about £80 It's actually quite tidily finished inside, giving confidence for a long useful working life..

It's now reassembled and I'm going to have a nice mug of tea
before I plug in and start breaking in the new speaker..

fingers crossed 20w heritage greenback will = everything from first 2 Steeleye Span LPs
to The Clash vintage guitar tones tones..