Batting conditions looked near perfect at the start of play and soon Lawrence, unbeaten on 105 overnight, was continuing his assault on the short boundaries.
What looked a leisurely flick off Craig Overton in the second over of the morning sent the ball flying into the James Hildreth Stand for six.
Surrey lost their fifth wicket on 341, Ben Foakes falling leg-before to Overton without adding to his overnight score of 64.
It was 362-6 when Mahipal Lomror was bowled off an inside edge by Lewis Gregory for ten.
Lawrence appeared to be playing effortlessly. He had faced 163 balls and hit 14 fours and five sixes when taking one chance too many and falling to a diving catch by wicketkeeper James Rew, chasing a wide ball from Gregory.
After 110 overs the scoreboard read 378-7, indicating three bonus points for Surrey and two for Somerset. Thomas ensured there was no respite for the home bowling attack, taking three fours off an over from Migael Pretorius to raise 400.
Dan Worrall provided solid support in a 36-ball innings that saw him contribute just five runs before shouldering arms to Leach and being pinned lbw by a ball arrowed into him by the left-arm spinner from around the wicket.
Reece Topley (seven) struck Leach for a straight six before being caught by Overton at first slip to give the England bowler a third wicket. Chahar, who had retired to the dressing room briefly after making two, returned to add a single to his score before being last man out, bowled by a turning delivery from Leach.
Somerset faced three overs before lunch and reached the interval safely with one run on the board. The afternoon session saw Lawrence strike an early blow with his off-spin, Josh Thomas, on 17, falling lbw pushing forward defensively.
Tom Kohler-Cadmore smacked Lawrence for a straight six in moving briskly to 22 before driving a ball from left-arm spinner Lomror in the air straight to Thomas at mid-wicket to make it 68-2.
Lammonby and Henry Nicholls added 29 before the latter fell leg before to Worrall on the back foot for 15. Lammonby went to his half-century, off 94 balls, with seven fours and was unbeaten on 61 at tea, with his side 126-3.
It was announced that Chahar had been forced to withdraw from the match, Ralphie Albert being introduced as an injury replacement and Lawrence’s place in the game was then taken by Nik Gorantla.
Two spinners down, Surrey were unable to prevent Lammonby and Abell raising a century stand, the latter going to fifty off 96 balls. Lammonby then spent an anxious few balls on 99 before a single to square leg off Lomror brought huge relief and sustained applause from another sizeable County Ground crowd.
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