Hi loco,
You may want to look through the Heljan product section of this forum site as it could be that you have a faulty class 47 chassis. Some were made from contaminated castings that subsequently grew in size over time and made it difficult to remove the body and eventually cracked the plastic bodies.
If, after investigation, you suspect you have a faulty chassis then Howes are really helpful regarding replacements - I got one replaced last week, no trouble at all.
Regards
Graham