I seem to remember an old episode from when I was young where the Dr and his companion (sorry can't remember which maybe Tom Baker era) got lost in the TARDIS.
The inside of the TARDIS was not an old hospital or anything like that, but was very much filmed in a studio and I seem to remember it being almost Roman architecture with maybe some gardens and a fountain.

Can anybody help with this?

Sounds like "The Invasion of Time".

From the description, I would say either The Invasion of Time or Logopolis (the Cloister room scenes).

Sounds like Logopolis to me.

I was just thinking about "Logopolis". Mind you, the b&w scenes from "Warriors Gate" also sprung to mind.

It was definitely in colour

Both Invasion of Time (1978) and Logopolis (1981) contain scenes like this.

Invasion of Time has several whole episodes given over to Sontarans chasing the Doctor and co round the Tardis. Some of the Tardis rooms were filmed on location (disused hospital as Tardis sickbay), some in the studio. The Tardis power room is an art gallery in disguise.

Logopolis has some scenes in the Tardis cloister room (lots of pillars and plants), and lots of scenes in corridors. Tegan comes on board the Tardis and immediately gets lost and panicked.

Both of these are in colour, with Tom Baker's Doctor, and both are out on DVD.

I would say the story the OP is thinking of is definitely "Logopolis" rather than "The Invasion Of Time".

These are two Classic Who stories where a fair amount of the action takes place in the Tardis, but the only one where 100% of the action takes place in the Tardis is the two-part William Hartnell story "The Edge Of Destruction" (aka "Inside The Spaceship").

This quite accurately describes my memory, I think it must be this.
I definitely watched Logopolis as a child.

It could be a combination of two memories - perhaps the story of Invasion of Time getting mixed with the visuals of the Cloister Room in Logopolis?

Since the OP said it wasn't in a hospital and it was filmed in studio, that would seem to eliminate The Invasion Of Time which was filmed on location at a hospital so therefore Logopolis seems to the story the OP is thinking off.