No, not Xara --- I finally managed to remember:

Cerilica (company name) Truism (product name).

The last big RISC OS program written in BASIC and assembler. I published something through Cerilica called TextFX, also in BASIC and assembler.

There was also Composition, by a chap from New Zealand. Multiple transparent layers of any size and position, and and interesting system-wide plugin system.

That just makes me sadder that MacBasic wasn't available and more interested in trying RISC OS on a Raspberry Pi....