http://www.southwesttrains.co.uk/fever.aspx

South West Trains is running a "Summer Fever" promotion during weekends
in August (plus Sunday 1 Sept) - on Saturdays, Sundays and the Bank
Holiday Monday, a Day Return is £15 (£7.50 for children) across most of
the SWT network - the exception being for journeys to/from all stations
between Exeter St Davids - Feniton and Warminster - Bristol Temple Meads.

You can book tickets at the station on the day, as well as online of
course. No mention of Railcards as such in the T&Cs, but a mock booking
via SWT's site doesn't show any extra discount.

As to be expected, it's only for travel on SWT services.

The Island Line is apparently included, and the T&Cs states that
"Discounted cross-Solent tickets to/from the Isle of Wight are available
from Hovertravel and Red Funnel ticket offices in conjunction with this
offer."

I'm not quite sure how that all works - doing a mock booking on the SWT
site of a Waterloo to Shanklin return journey, one isn't offered a
"Summer Fever" fare, even with a supplement. Maybe from a ticket office?

There's no mention of the offer that I can find on the Red Funnel
ferries website, however it does feature on Hovertravel's website...
http://www.hovertravel.co.uk/offer/South-west-trains-summer-fever/

The important bit:
"[...] show your [SWT Summer Fever] ticket at Hovertravel and you will
also receive a discounted day return for £10 per adult & £6 per child!"

If anyone knows whether (and if so, how) one can cobble together a
mainland-island (or v.v.) through rail journey with this offer that's be
most appreciated.

The mysteries of getting to/from the IOW aside, given SWT's fares are a
bit on the pricey side, this is a decent offer - Waterloo to Weymouth
for £15 return, not bad. Try not to fall asleep on the beach.