If the recipient is on a UK phone roaming (successfully) in Canada it
costs no more for you to text them (you're texting a UK mobile), and AFAIK
no UK network charges for receiving texts when roaming. Certainly Orange
and Virgin don't.  They'll pay more to reply though.

Not quite. He was asking how much to text a UK mobile that was roaming in
Canada. Answer is, same as if the recipient was at home.

<snip>

Whereas if we'd actually read the OP, he states that his mate in Canada is
on a UK network, so the real answer is ..... the same as it costs to text
him when he's in the UK!

Boy, that doesn't sound right.  With my T-Mobile PAYG account incoming
text seems to be free everywhere it works at all.  I also know it roams
at 55p/minute for voice in the USA and Canada, and from the web page
is UKP1.40/minute for voice in Brazil.  And if you are getting voice
at 50p/minute in Trinidad, including outgoing overseas calls, you may be
doing better than even a local PAYG SIM; calling costs in Trinidad
are horrible, and the T-Mobile web page calls it UKP1.40/minute for
PAYG roaming there.

In any case, it costs the person who is roaming to receive a voice call
and it is expensive enough that the average person who is paying their
own bill is unlikely to want to chat for a long time when someone
phones them.  I wish roaming were cheaper; right now I have a collection
of local PAYG SIMs for countries I visit often enough to keep them
alive, and I keep a London geographical number along with Hong Kong
and USA mobile numbers alive where I am by forwarding through Skype
to the local number.  Some places this works really well.  I spent
3 weeks in China last month getting calls that cost me 1.2p/minute
and were cheap local calls to most of the callers.

Dennis Ferguson