Revised answer after further research.
London TravelWatch is the independent, statutory watchdog for transport users in and around London. In these consumer affairs committee meeting notes we can read the following:
2.4 To check the legal position for passengers
The Director, Public Liaison reports that the ticketing contract
entitles you to travel between the starting point and destination. It
does not specify whether the service is a railway service or a
replacement bus but the operator should advise you in advance if it is
a bus if possible.
2.6 The rights of passengers in event of breach of conditions of service
“Passengers on rail replacement services are entitled to the same
conditions as these travelling on normal rail services. Therefore they
are entitled to delay repay in the event of delay. The EU Regulations
on rail passengers rights and obligations if enacted, will strengthen
the rights of passengers to be provided with assistance and
refreshments in the event of delay and full refunds in the event of
severe delays leading to cancelled trips.“
The National Rail Conditions of Carriage state:
60. Carriage by road vehicles
These Conditions apply to passengers travelling and luggage, articles,
animals and cycles conveyed in road vehicles provided by a Train
Company or its agents unless it is made clear to you when you buy your
ticket that different conditions apply. The term “train” in this
context includes any road vehicle owned or operated by a Train Company
or on its behalf.
Individiual conditions of carriage may override the above and a page with pointers to the individual train operating company's CoC can be found here. Some parliamentary comment can be noted on the lack of standardisation:
There is also a complete lack of standardisation. The only common
element of Delay Repay schemes across the train operating companies is
the 30-minute threshold for compensation—everything else differs. The
circumstances in which compensation can be paid vary; some
compensation schemes include the weather and planned engineering
works, whereas others exclude them, so there is no clarity about what
passengers will get.
Although there is a typical exclusion in the various CoCs stated as:
Delay Repay is a national scheme train companies use to compensate
passengers for delays. You can claim compensation if you're delayed by
more than 30 minutes when you travel with us (except when delays are
caused by planned engineering work).
This appears to be related to delays relating to the original scheduled time when the timetable has been amended due to delays, e.g. from the UK Citizens advice bureau comment on engineering works :
If the train company has given advance warning of engineering works
and altered the timetable you will not normally be entitled to
Virgin Trains gives this clarification:
When a specially-advertised revised timetable is in operation, for
example at times of engineering work, we will be measured against the
Anecdotally some argument exists as to whether this is an absolute get-out clause, e.g. from a popular UK site discussing railway issues:
Yesterday I travelled from St Pancras to Rainham by Southeastern. Due
to engineering works the hourly morning train service was running to a
special timetable with departures at xx55 rather than the xx25 in the
normal timetable. As a result I wasted 30 minutes at St Pancras and
arrived at Rainham 30 minutes later than I had expected.
Would this be claimable via delay repay?
No, a timetable change for planned engineering works is not a delay.
No not at all. Because of the nature of the bus replacement services
then there is no case for a refund.
Again the delay repay is not applicable when there are engineering
works in force
The appropriate advice would appear to be to study the precise conditions of carriage of your train operating company backed by the above additional information and claim for delay repay anyway. It can't hurt to claim.