Britain's FTSE slips, though posts fifth straight month of gains

* FTSE 100 down 0.6 pct

* Longest monthly run of advances since 2013

* WPP (LSE: WPP.L - news) jumps after update

* Shire (Xetra: S7E.DE - news) and Next (Other OTC: NXGH - news) fall ahead of earnings (ADVISORY- Follow European and UK stock markets in real time on the Reuters Live Markets blog on Eikon, see cpurl://apps.cp./cms/?pageId=livemarkets)

By Kit Rees and Alistair Smout

LONDON, Oct (Shenzhen: 000069.SZ - news) 31 (Reuters) - Britain's top share index retreated on Monday but posted a fifth straight month of gains, as advertising group WPP rose after reporting its third-quarter results.

The blue chip FTSE 100 index was down 0.6 percent at 6,954.22 points by the close, but up 0.8 percent for October.

Shares (Berlin: DI6.BE - news) in WPP rose 4 percent and posted their biggest daily gain in four months. WPP, the world's largest advertising group, reported results in line with expectations .

"We are encouraged by the positive nature of this morning's update and remain fundamentally positive on WPP's ability to capitalise on a solid medium-term outlook for global advertising spend," Roddy Davidson, an analyst at Shore Capital Markets, said in a note.

However, investors were less optimistic about some forthcoming earnings. The top fallers were Shire, which reports results on Tuesday, and Next, whose results are due on Wednesday, down 2.8 percent and 3.2 percent respectively.

The FTSE 100 index posted a fifth month of gains in a row for the first time since early 2013. The index has been buoyed by a rally in banks and mining companies.

Banks were boosted by last week's well-received earnings from Barclays (LSE: BARC.L - news) and Lloyds, with the FTSE 350 banking sector hitting its highest level for the year.

October saw the FTSE 100 set a record high of 7,129.83 points. A weaker pound has bolstered the index, which has rallied about 10 percent since Britain voted in June to leave the European Union. The cheaper pound helps the index's international, dollar-earning firms.

Some analysts, however, were more cautious on the outlook for British shares.

"(The FTSE) is still continuing the positive run of gains since Brexit, so it's a Brexit bounce. But it's run into that previous record high ... and pulled back quite significantly from there," said Jasper Lawler, market analyst at CMC Markets (LSE: CMCX.L - news) .

Investors, he added, were holding back before upcoming central bank interest rate decisions and the U.S. presidential election on Nov. 8.

"I think no one's really got the confidence to buy the market up through into new record highs."

The more domestically exposed FTSE 250 index fell 0.6 percent, and was down 1.8 percent for the month of October. It has been weighed down by a spate of profit warnings and big declines from companies including Berendsen (LSE: BRSN.L - news) , Cobham (Amsterdam: CH6.AS - news) , Senior (Other OTC: SNIRF - news) and Keller. (Editing by Mark Heinrich)