* Acquires further 13.6 pct in Gladstone for 2.2 mln stg
* Offer values entire co at 18.1 mln stg
* Offer at 17.9 pct premium to Gladstone's Monday close
* Gladstone shares up 12.5 pct (Adds details)
March 2 (Reuters) - Canada's Constellation Software Inc (CSU.TO) said on Tuesday it would make a mandatory cash offer for British software firm Gladstone Plc GLAD.L at 33 pence per share after raising its stake in the company to 43.5 percent.
Constellation said it acquired a further 13.6 percent of Gladstone, or 6.6 million shares, for 2.2 million pounds ($3.3 million).
The 33-pence-a-share offer, which represents a premium of 17.9 percent to Gladstone's Monday close on the London Stock Exchange, values the existing issued share capital and in-the-money share options of Gladstone at about 18.1 million pounds.
"Gladstone represented an opportunity to establish a position in the UK health club and leisure centre sectors and Constellation acquired a substantial shareholding in Gladstone in March 2008," Constellation said in a statement.
Gladstone shares were up 12.5 percent at 31.5 pence at 1540 GMT, while Constellation shares fell 10 Canadian cents to C$39.90 on the Toronto Stock Exchange. ($1=.6670 Pound) (Reporting by Tresa Sherin Morera in Bangalore; Editing by Unnikrishnan Nair)