I am planning a day/weekend down the southcoast to get these units, just as long as i can get cheap tickets.
Get yourself a Freedom of Severn and Solent 3 in 7 Rover, £40 (£26.50 with a railcard, a bit less for a child one), valid from Cardiff Central right down to Lymington Pier and much more. Not sure how the Off Peak Return compares from Cardiff Central to Lymington Pier, but I would reckon it's more than that. I'll have a look. Not sure either of course which part of South Wales you're coming from mind, but getting to/from Cardiff or Newport shouldn't be too much (I would hope!). The rover, however, isn't valid until after 0930 weekdays (all day Saturdays, Sundays and Bank Holidays) but worth getting because at least you're then valid to stay on the slammer all day long!
I'm considering another trip to scratch in the slammers again, not been on them for a fair while. Might do it sooner than later, even possible linesiding for some different photos!
I do wonder whether SWT can manage to get another 450 down there, and whether they'll bother doing any special runs with a 73 hauling the slammer before the end (wishful thinking, I know)!
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To the OP:
Cardiff Central to Lymington Pier Off Peak Return is £39, with Advance singles from £13 each way. With a Y-P, it's £25.75 and £8.60 respectively. Obviously, I can't tell you the exact fare from where you are, but on the above example it's only a £1/75p difference between the Off Peak Return and the FoSS, which gives you plenty of flexibility (I doubt you'd need to return beyond the validity of the rover) and as said before you've also then got the chance to go up and down plenty of times and cover other tracks down that way if you should so desire.