In a press release (link below) from SGX they say:

Clearing of financial derivatives

New OTC interest rate swap transactions cleared was S$8.1 billion notional in June, up 70% year-on-year.
Since the start of IRS clearing in November 2010, SGX has cleared a total S$273 billion notional of IRS.

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Converting to GBP at 2:1 means £4bn per month cleared.

http://www.sgx.com/wps/wcm/connect/sgx_en/home/higlights/news_releases/news+release+-+sgx+derivatives,+commodities,+clearing+volumes+up