Some insider info below.
Thanks to my contact...
"Matisa P95 Track Renewal Train system 2 – ie 78801, 11, 21 and 31 – is going home to Crissier next week for a 10-yearly overhaul.
It’s due to transit the channel tunnel on train 4450 which leaves Dollands Moor after midnight on Thursday, and will be travelling from Wembley on the connecting service that arrives at Dollands Moor at 21h30 on Wednesday evening. Not sure when it will make its way to Wembley, but current plans are for it to leave Sandiacre at 18h30 on Tuesday.
The train will be away for about 3 months, and is expected to resume duty in early October."