Finally have to admit defeat with yesterday's Times.
26a. Enormous old creature, female, trampled over me (10). I have D?M??R?D?N (assuming 21d is EMBED and 18d is HOISTER.
27a. Girl after going left all clues in separate places (4). I have E?E?
Thank you both for your help. Dimetrodon is not in Chambers, unusual for the Times. No wonder I couldn't get the last one - for some reason I had put in POLE instead of POLL.
I have no problem with 21d - From mid-September onwards has daughter finally bringing plant inside EMBED. I am less happy about 18d - one who's put up lives in hotel that's changed hands. HOISTER fits the wordplay but doesn't match the definition in Chambers.