Virtualmin

Whilst editing a previously setup virtual server here:

Virtualmin > Edit Virtual Server > Quotas and limits

and changing (increasing) the

"Total server quoto" & "Server administrator's quota"

limits from 5GB to 6GB. This did not actually change the quota limits. I had to manually go to the following two places:

Webmin > System > Disk Quotas > Users
Webmin > System > Disk Quotas > Groups

and change the particular user/group from 5GB to 6GB as these figures had not changed.