£4000 pound does sound excessive by at least £1000, and as mentioned above, bg have a reputation for charging ridiculous prices.
I have been fitting Worcester Bosch CDI's virtually exclusively for 5 years and all my clients are perfectly happy with them; only had one of them fail, and that was due to either lightning or pulse in the grid that fried the electronics. Was repaired free of charge the next morning, 2 months out of warrantee.
Vaillant are considered similar quality, but have very poor after sales service.
I have been fitting Worcester Bosch CDI's virtually exclusively for 5 years and all my clients are perfectly happy with them; only had one of them fail, and that was due to either lightning or pulse in the grid that fried the electronics. Was repaired free of charge the next morning, 2 months out of warrantee.
Vaillant are considered similar quality, but have very poor after sales service.