Centuries from Richard Montgomerie and Chris Adams are unlikely to save Sussex from only their second Championship defeat of the season some time today.

The county have been on the back foot from the moment Adams decided to bowl first on a flat track at Cheltenham College.

Gloucestershire duly cashed in by making 520 and after Sussex had been bowled out for under 200 for only the second time this season in their first innings, they followed on 359 behind.

While Montgomerie and Adams were putting on 126 for the second wicket yesterday afternoon there was just the chance that Sussex might extricate themselves with a draw, or at least inconvenience Gloucestershire by making them bat again.

But Montgomerie fell seven runs after reaching his sixth hundred of a prolific summer and the game swung decisively away from Sussex in the first over after tea when Bas Zuiderent and Michael Yardy were removed by Ian Harvey in the space of three balls.

In just four overs Harvey wrecked Sussex's middle order with 3-4 before Adams, who finished unbeaten on 121, and Mark Davis dug in to take Sussex to 315-6 at the close, still needing 38 to make Gloucestershire bat again.

To add to Sussex's disappointment, and that of the 300 or so of their supporters who have made the pilgrimage to the most beautiful ground in the country, Middlesex's innings win over Durham means they have taken over the leadership of the second division.

The sides meet at Hove on Thursday and coach Peter Moores says they may freshen up the side.

Certainly one or two of the more inexperienced players appear to be feeling the effects of a gruelling county campaign.

For six of them the rigours of an English season is something they are not used too and there has been evidence in this game that it's starting to show, if not physically then mentally.

On Saturday they tumbled from 112-2 before being dismissed 25 minutes into the third day's play for 167 and yesterday they lost four wickets in six overs after tea when they subsided from a relatively prosperous 214-2 to 244-6.

Harvey initiated the collapse when he softened Zuiderent up with a short ball which he fended off with the glove. When the next was pitched up, the Dutchman drove without a great deal of care and attention to Mark Alleyne at second slip.

Another short delivery accounted for Yardy who fended his second ball off the glove and gave Alleyne more catching practice.

Adams, anxious no doubt to atone for his mistake at the toss, must have feared he would run out of partners before reaching his hundred. Of the 27 runs put on for the fifth wicket he contributed 24 while at the other end Umer Rashid, clearly intimidated by Harvey's aggressive approach, tried to cut James Averis but only succeeded in edging to Jack Russell.

Harvey collected his third wicket when Matt Prior drove on the up to gully, prompting most of those Sussex supporters to get on their phones and cancel hotel bookings for last night.

But Adams found the ideal partner in the obdurate Davis who has never given his wicket away cheaply.

They repelled Harvey to put together an unbroken seventh wicket stand of 71 although Adams was dropped at short leg off Martyn Ball when he'd made 111.

Adams' hundred took exactly four hours, which is relatively slow by his standards. Indeed his first 50 came off 144 deliveries, but he accelerated to score the next 50 at a run a ball, 46 of them coming in boundaries and most hit with brutal power in his favourite areas between point and mid off.

While the youngsters might be feeling the pace, the experienced core of the Sussex side show no signs of flagging.

In his current form, Montgomerie must have been disappointed to miss out in the first innings on such a good wicket, but there was never much chance that he would make the same mistake a second time. There was some slow turn to assist Ball and Jeremy Snape.

But Montgomerie used his feet well to find the gaps and anything remotely off line from the quicker bowlers was dispatched with the ruthless efficiency Sussex supporters have come to expect from him in recent weeks. He'd made 107 when Ball, diving low to his right, took an excellent return catch off a mis-cued drive. Montgomerie hit 19 boundaries and faced 173 balls and his first-class aggregate for the season is now a hefty 1,254.

Montgomerie had put on 49 for the first wicket with Murray Goodwin who seemed to be on the end of his second poor umpiring decision when, well forward, he was ajudged lbw to a ball from Alleyne that brushed his thigh pad.