Grandfather rights are removed when you reach 70 years. Also I was told that the competence test for MH's has not yet been introduced by the UK
Don't think that is strictly true.
At 70 your licence expires and you can either renew at the lower 3500kg limit and surrender your GF rights or you can renew to maintain your C1+E etc up to 7500kgs/ 8250gtw GF rights by undertaking a medical and eye test etc which if passed and renewed will be good for a further 3 years.