On May 11, 6:40*pm, wrote:
First of all, thanks all of you for your suggestions. Especially Barry
who looks as though he has discovered the most cost-effective. Today
is 8 May and I will be putting it to the test in about an hour. I was
SNIP


Sorry, today is 11 May, I want it to be valid from 12 May as I have
paper for today (see earlier post). So hopefully it will cost me even
less!

Credit for old ticket: £2.65 x 233 days = £617.45
Charge for new ticket: £3.11 x 233 days = £724.63
Difference to pay: £107.18

Then as before, if I change back to Z12 only from 28 July, it'll be
156 days validity,

Credit for old ticket: £3.11 x 156 days = £485.16
Charge for new ticket: £2.65 x 156 days = £413.40
Refund due £71.76, in other words the period from 12 May to 27 July
inclusive will have cost me £35.42

Thank you thank you thank you Barry, fingers crossed that I will not
be met with blank looks when I get to Moorgate station in a moment :-)

I'll let you know tomorrow.