Clutch judder 1/2hr into drive

Help Please !
2002 1.8i Freelander ESX.
Engine replaced at 34k due to head gasket. (2007)
New clutch fitted at the same time.
6 months on and clutch release fine from cold but as the journey progresses a clutch judder slowly emerges. Nightmare when caught in Bank Holiday motorway crawl. Clutch release near floor.
Took into garage but as they tried it from cold there was no problem found.

clutch

whats one of them

New clutch fitted at the same time.
6 months on and clutch release fine from cold but as the journey progresses a clutch judder slowly emerges. . Clutch release near floor.
Took into garage but as they tried it from cold there was no problem found.
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sounds like it could be hydraulic, did they change the master/slave assembly as well?
Especially as its when its hot...maybe that could be the answer .

Also when the clutch was done was it the whole lot that was changed ...or just the plate?
Hope this helps
D