[ic] £ or £ for UK currency symbol in Locale

[John1]

We operate an interchange site that sells in UK pounds.

On a few rare occasions customers complain to us that all prices on the
website have a ? instead of a £ sign in front of the prices.

In Locale we have set currency_symbol to £.  Consequently the html for our
pages contains the £ symbol rather than £

Could this be the reason for some people getting question marks instead of
pound signs?  i.e. Could the question mark be due to the regional settings 
of their computer or a lack of installed fonts?

If so, should we change currency_symbol in Locale to £ instead of £

I am a little cautious about doing this as it would also impact the plain 
text e-mail templates which use the [currency] tag.  i.e. They would display 
£ instead of a pound sign, so I would have to change any e-mail 
templates to hard code a pound sign.

Can anyone out there confirm whether or not I should change currency_symbol 
to £ and proceed as above, or would this not solve the question mark 
problem anyway?  Thanks.