There is. Lots of seat reservations from Brighton - Bristol ive seen put onto the services, as well as from other stations along the line before Cosham. Service regularly leaves full up too.
The train is generally very busy departing Brighton simply because it is the only service that isn't a deathly slow Southern! Many of those people are off by Havant and Fareham, or indeed well before. The service does sometimes carry a good load of leisure travellers, but a great many of them have Advance tickets and pay peanuts. On many occasions though the train is dead from Fareham or Southampton onwards, and most crew will tell you it's an easy job. By the time you get to Bristol you could often fit the loadings into a minibus.
The delays on the inward run (1O98) can occur anywhere, but once they've happened they are exacerbated considerably when the train gets onto the Coastway. Every other service is a Southern that stops at every pile of bricks, and there is little opportunity to get past whatever might be in your way. It tends to start at Southampton, the service often ends up behind a Southern timetabled not too far behind it. If the Sprinter is right up his backside, the panel will usually loop the Southern at Northam, but if that doesn't happen it's a slow run all the way to Fareham and beyond, as the cucumber also stops at Swanwick. Once you get to Havant it has then got to vie with everything coming out of Pompey for a gap, which of course includes more Sussex-bound stoppers as well as SWT. It can slip past at Barnham or Worthing, but only if there's room to get you round. By then though the decision already needs to have been made, and it's often terminate at Barnham. Barnham tends to be preferable to Worthing as there is a siding there rather than having the unit sat in the loop platform.
The appearance of 150 units is certainly not ideal, but the current policy appears to be the use of either of the pair of three car 150/9 units (a 150/1 with a spare /2 car in the middle) as first choice substitute when a 158/9 is unavailable. Whilst this provides seats, it does nothing for pax comfort!
....but it's the most recent thing I could find about the Brighton stats. What it does show though is that the unreliability of this service has been ongoing for a while now......
