You need to look for an installer that is listed as a Trustmark  B13 Park Home Insulation Installer.
Nobody else can provide the work with the necessary guarantees and insurances to allow you to get the grant voucher.
All installers working on insulating Park Homes MUST HAVE Trustmark registration under PAS2030:2019 part B13 : Park Home Insulation Installer and follow PAS2035 standards.
(At present CIGA is the only Trustmark Scheme Provider that is providing Trustmark Registration for Park Homes Insulation Installers.)