* FTSE 100 rises 0.8 pct, touches highest since Sept 2014
* Index rallies after new ECB policy
* UK stocks underperform peripheral Europe
* Royal Mail gains after update (Recasts)
By Alistair Smout
LONDON, Jan 22 (Reuters) - Britain's top share index touched a four-month high on Thursday after the European Central Bank launched a massive sovereign bond purchase programme to fight deflation and spur growth.
The ECB said its new quantitative easing programme and existing asset purchase schemes will pump 60 billion euros a month into the euro zone economy from March until Sept. 2016.
The move boosted appetite for shares across Europe and Britain's FTSE 100 touched a peak of 6,797.01 points, its highest since Sept. 22.
"The higher than expected monthly amount to be purchased is compensated (for) by the time limit on the programme," said François Savary, chief strategist - head of asset services at Swiss Bank and fund management group Reyl & Cie.
He described it as a "great compromise", adding "at the end of the day Mr Draghi and the supporters of the QE policy move must be quite satisfied".
But Britain's top share index pared gains to trade 0.8 percent higher at 6,782.10 points by 1526 GMT, as a stronger dollar hit the price of oil and related stocks.
British exporters also lost competitiveness as the euro weakened against sterling, and the FTSE 100 underperformed peripheral euro zone markets that stand to benefit from QE, with Spain and Italy, up more than 1 percent.
"The euro came down, the dollar rallied, and that scuppered FTSE outperformance. The index is only an indirect beneficiary of this policy," CMC Markets analyst Jasper Lawler said.
Expectations the ECB would start printing money have fuelled recent gains, with the FTSE over 3 percent higher this week.
While investors welcomed the move, some were sceptical about how quickly its effects would be felt.
"The bare outlines are broadly in line with what the speculation has been over the last couple of days," said Andrew Milligan, head of global strategy at Standard Life Investments.
"But on what terms will we see a sustained adjustment in the path of inflation? We have to see a response in the real economy."
Among individual movers, Royal Mail rose 3 percent after saying group revenue for the nine months to Dec. 28 rose 1 percent, a slowdown from the half year as an improved parcels performance was offset by lower letter volumes.
British insurer RSA rose 3.1 percent to 466 pence after Credit Suisse raised its stance to "outperform" from "neutral" and pushed its target price to 525 pence from 450. (Editing by Catherine Evans)