A useful site if you have not already found it is
http://smart.com.ph/Account/Default....d/buckets.aspx
if you register on there you can check your balance for free for both your Smart SIM's (the Plug n Talk and your Roaming phone).
It's also worth bearing in mind that it can be a better deal to ask someone in the Phils to reload your numbers rather than using the UK reload services, obviously only works if you are sending other cash to the Phils but it is a lot cheaper if you are using the phone regularly.