Essex close on 271-6 as Somerset fight back after tea
Essex opener Cook out for brilliant 172
Somerset made 301 all out in first innings
Team with best first-innings score wins trophy if game drawn
Cook 103, Westley 38
More anguish for Craig Overton as Tom Westley picks up four through the vacant third man area.
It was a false shot, but all along the ground. Those are the only runs off the over.
Essex 151-1
Image source, Getty ImagesThere it is!
An 18th four from his 164th ball brings Alastair Cook his 67th first-class century.
It's an absolute gift of a full toss too from Jack Leach as he hammers it away to the point boundary for his seventh ton on this ground.
All his Essex team-mates on the dressing room balcony, plus most of the assembled press, stand to applaud a fine innings.
Image source, Getty ImagesCook 99, Westley 34
One run away now for Alastair Cook as he picks up a couple more singles into the legside.
Tom Westley also nudges another run too. That's now his joint-highest score in this competition this season.
Cook 97, Westley 33
Two more for Tom Westley off that Jack Leach over as he glances it neatly fine on the legside.
Alastair Cook watches on contently from the non-striker's end as he prepares for Craig Overton to return from the Pavilion End.
Cook 97, Westley 31
Two more fours for Alastair Cook in the space of three Jack Brooks balls and he is now just three away from a century.
Square drive and then a cut shot. Dismissive batting.
Cook 89, Westley 31
Image source, Getty ImagesMaiden over to start with for Jack Leach.
He forces a rare false shot too out of Tom Westley as he tries to cut the ball away, but takes a swish and a miss.
Vic Marks
Ex-England spinner on BBC Radio 5 live Sports Extra
For the first time Somerset are looking rather ragged in the field.
Image source, Getty ImagesIt's taken 46 overs - but Jack Leach is finally going to come on to bowl for Somerset from the Nursery End.
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Paul Atkinson: With so many catches falling short, why doesn't the slip cordon move in a bit? If one gets big on you, you've still got a chance of taking it, but if it doesn't reach you, it's a chance gone altogether. If they're not carrying, get nearer so they do.
Simon Kinsler: Alistair Cook was run out against India (2012 I think) 10 runs short of a double century when he took evasive action from a throw. He lifted his bat out of the crease and the ball hit the stumps.
Cook 89, Westley 31
Alastair Cook looks like he's playing a different game to everyone else right now.
Periods of quick scoring and then more than happy to go back into his shell and bide his time for a few overs.
Cook 87, Westley 30
Full and wide outside off from Josh Davey to Alastair Cook. Four more.
Rinse. Repeat.
Two balls later - same result. Runs flowing now, and that's the century stand between Cook and Tom Westley.
Vic Marks
Ex-England spinner on BBC Radio 5 live Sports Extra
I don't think Somerset have bowled too badly, it's been a challenging contest, they haven't made it particularly easy for the Essex batsmen but Cook has been extremely good, there's no surprise there.
Westley, happy to play the secondary role but if anything is on his legs he's clipped it away gratefully.
It's a serious match this and Essex are batting in a serious manner, they're not racing along but this is a significant partnership now.
Cook 79, Westley 30
Jack Brooks' turn to be driven for four through the covers by Alastair Cook.
Far too full and wide from the Somerset seamer at the start of a new spell.
That's 13 fours now for Sir Alastair.
Image source, Getty ImagesSomerset a bringing a Jack on to bowl....
It's Brooks though instead of Leach from the Pavilion End.
Cook 74, Westley 30
Two more runs tucked away for Alastair Cook off Josh Davey down to the fielder on the deep square leg boundary.
Good running between the wickets as well, which Vic Marks has just conceded at the back of our box may well be Somerset's best chance of getting him out.
Another exquisite cover drive for four follows to take Cook into the 70s.
Cook 68, Westley 30
Lewis Gregory beats Alastair Cook's outside edge with a ball that nips away a touch down the slope.
But, Cook replies with a back foot drive through the covers next ball for four and then another neat glance for three through mid-wicket.
Big appeal off the last ball against Tom Westley off the last ball from a full inswinger but not out is Rob Bailey's reply.
A timely reminder here from our colleague Mr Mountford of how you can follow this match across BBC Sport...
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Varun Chopra
Essex batsman on BBC 5 Radio Live Sports Extra
The batting seems a lot more calmer and easier after lunch, we've seen how crucial it was getting through that new ball period with minimal damage and now the runs look to be coming a little bit easier.
Essex trail Somerset by 197 runs
Josh Davey finds an edge!
But Tom Westley survives as a chance falls short of Craig Overton's left hand at second slip. Hands on heads once again for Somerset.
Maiden over from Davey.
A bowling change for Somerset....
It's not Jack Leach though. Josh Davey replaces Craig Overton from the Nursery End.