I am very suprised that they issued the Visa with such little time left on the Medicals.
When my son was asked to submit his passport, we were told he could defer and get another medical if he didn't think he could travel to Canada in time to activate the visa. At the time, he had 5 weeks before his medical expired.
I am presuming your daughter wasn't asked if she wanted a deferment?
This is a copy of the e-mail we received for his passport request:
Dear ******
Please be advised that your file is now ready for visa issuance.
Please find attached instructions for submission of your passport.
Once issued your visa will be valid until 10-03-2010. You must enter Canada on or before this date.
If you cannot submit your passport immediately and make arrangements to land in Canada before the expiry of your visa, a new medical examination will be required. If you choose to submit your passport by post, please send it to the address below:
PUP Unit - URGENT
Immigration and Medical Services Division
Canadian High Commission
38 Grosvenor Street
London W1K 4AB
I would definitely contact CIC and explain that the passport with the visa was sent AFTER the expiration of the Visa - hopefully you have proof. I would send a fax with all the details and a copy of the letter and envelope showing date of postage, and the COPR Visa showing date of expiry.
If you go to the London website, there is a list of phone numbers, fax numbers and CIC Representatives - I would suggest you to speak to someone before you send the fax. The list of representatives includes CIC officers, First Secretaries, Second secretaries etc.
http://w01.international.gc.ca/cra-rce/mission.aspx?lang=eng&MID=36Telephone Immigration (011 44 20) 7258 6699
Fax General Fax (011 44 20) 7258 6333
High Commissioner Mr James Wright
Deputy High Commissioner Mr C. Boucher
Minister Ms A. Arnott
Counsellor (Immigration) Ms N. Scott
Counsellor (Immigration) Mr J. Maffett
First Secretary (Immigration) Ms S. Sweetman-Griffin
First Secretary (Immigration) Ms S. Sweetman-Griffin
First Secretary (Immigration) Ms M. Purdy
First Secretary (Immigration) Mr G. Greenfield
First Secretary (Immigration) Ms L. Zadravetz
First Secretary (Immigration) Ms S. McKale
First Secretary (Immigration) Mr R. Stevenson
First Secretary (Immigration) Mr G. Stewart
First Secretary (Immigration) Mrs J. Lisko
Second Secretary (Immigration) S. Lee
Second Secretary (Immigration) Mr C. Raymond
Second Secretary (Immigration) Ms M. Sessini
Whilst normally expiration of a visa would entail re-applying one would hope that CIC are able to re-issue when there has been an obvious oversight.
Keeping my fingers crossed for you!

Edit: There is a news update on the website:
Phone service to the High Commission of Canada in London has been affected by the fire at the British Telecom central office on March 31 2010. Until service is restored, the High Commission of Canada can be reached at 020 7451 0800. I don't know if the Immigration line has been similarly affected.