Custom Splash Screen not Displayed

[Charles]

I have installed a custom splash screen using grub-mkconfig which works if secure boot is disabled. However, when secure boot IS enabled the usual purple splash screen is displayed and when c is used to enter the command line from grub's boot menu the system complains when any valid command is entered that secure boot does not permit this.

Release: Description: Ubuntu 14.04.3 LTS
                    Release: 14.04

Package: plymouth:
                        Installed: 0.8.8-0ubuntu17.1
                        Candidate: 0.8.8-0ubuntu17.1
                        Version table:
                       *** 0.8.8-0ubuntu17.1 0
                                    500 http://gb.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
                                    100 /var/lib/dpkg/status
                               0.8.8-0ubuntu17 0
                                     500 http://gb.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: plymouth 0.8.8-0ubuntu17.1
ProcVersionSignature: Ubuntu 3.19.0-33.38~14.04.1-generic 3.19.8-ckt7
Uname: Linux 3.19.0-33-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.18
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Nov 17 21:04:20 2015
DefaultPlymouth: /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth
InstallationDate: Installed on 2015-11-16 (1 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
MachineType: ASUS All Series
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.19.0-33-generic.efi.signed root=UUID=8553c46f-8672-42c2-8b25-d5e44aadf3d2 ro quiet splash vt.handoff=7
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
ProcFB: 0 nouveaufb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.19.0-33-generic.efi.signed root=UUID=8553c46f-8672-42c2-8b25-d5e44aadf3d2 ro quiet splash vt.handoff=7
SourcePackage: plymouth
TextPlymouth: /lib/plymouth/themes/ubuntu-text/ubuntu-text.plymouth
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 01/28/2014
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1402
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: MAXIMUS VI HERO
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: PCS
dmi.chassis.type: 3
dmi.chassis.vendor: Chassis Manufacture
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1402:bd01/28/2014:svnASUS:pnAllSeries:pvrSystemVersion:rvnASUSTeKCOMPUTERINC.:rnMAXIMUSVIHERO:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
dmi.product.name: All Series
dmi.product.version: System Version
dmi.sys.vendor: ASUS

[Charles (c-cross7)]

I have now copied everything to my laptop which works in secure mode, so presumably it is something to do with keys missing from my desktop. Not sure whether this still counts as a bug or not, but I would appreciate any help you can provide.