Q:

Which British author wrote about Shellworlds before Iain M. Banks?

Iain M. Banks is remembered as creator of Shellworlds-artificial planets with layers of habitable space within them. 
Except this is incorrect. I distinctively remember reading a British author in my youth that had a whole SF series dedicated to this very idea. One of the titles was something along Invasion from the Centre.
Does anyone remember who the author might be?

A:

Brian Stableford, Journey to the Center (DAW Books, 1982), followed by Invaders from the Centre (1990) and The Centre Cannot Hold (1990).
The trilogy is also published as the Asgard Trilogy, as Asgard's Secret, Asgard's Conquerers and Asgard's Heart. 

The galaxy is full of mysteries. Asgard one of the most inexplicable. Many different species & many types of individual sought answers to its enigmas. Anthropologists, fortune hunters, scientists, gangsters, politicians & explorers found reason to congregate on the planet which possibly wasn't a planet. To stand on the artificial surface & wonder how many subcutaneous levels the planet contained, & what, if anything, was at the center, to examine the artifacts on the reachable levels where the temperature approached absolute zero.

