Thanks for the reply / answer.
By chance, I may be travelling on a 159 & 158 tomorrow as all being well, I am travelling around part of the SWT network. 0920 Waterloo to Salisbury, connecting to the SWT service onto Southampton, then on to what ever electric unit they are using to Portsmouth & SS, probably a blue desiro, wait there for 20 mins for another SWT to Portsmouth Harbour, then a blue or maybe awhite desiro to Waterloo.
I do recal several years ago travelling on a 2 coach SWT diesel unit from Brighton, (I got off it at Havant) but I think it was going to Basingstoke & onto to Reading.
SWT did have some services along the south coast and also from Portsmouth for a while some to B/Stoke & Reading, (I am not sure if these were then 158s or 159s, or perhaps something else as I do recal too a declassifield 1st class bit at the front.) SWT did have also a very infrequent service from Brighton to Plymouth, all now sadly gone, along with their servcies from Waterloo beyond EXD to Plymouth, Paignton and even once a week to Penzance.