Peter Frimberley wrote:

Well I don't know about whether you can change now. But the sums are
fairly simple using the fares on the TfL website. it's £3.50 before
7pm, £2 if you can travel later, so work out how many days a week you
can come home after 7pm and deduct accordingly. Assuming you come home
before 7pm every day then you're looking at £7 a day, or £35 a week on
prepay. Or, approx £455 for 3 months depending on how many working
days there are that quarter. Whereas you've presumably paid £435.60
for your 3 months travelcard.

It looks like I'm better of with what I've got in that case.

So prepay is approx £20 per quarter more expensive, but if you have a
few days off in that quarter, or come home before 7pm a few days,
you'd probably pull that £20 back. On the other hand you might go in
to town on a few weekends, which would be free journeys on the
travelcard. Only you can decide ...

A good way of putting it.

There is a nice calculator at
http://www.mediauk.com/content/oyste...calculator.muk
which does the sums over an annual period, but still gives you monthly
results, again like my calculation you need to decide whether you need
the travelcard on weekends, whether you might travel after 7pm, etc,
to work it out.

Very useful. Thanks very much. :-)