TURIN,
I believe VP959 to be 100% right. You can do whatever engine maintenance you like, and just need to detail it in the in the log book - no other signatures necessary.
However, one thing that has not been mentioned is the requirement for parts - I'm not sure on engines, but any airframe parts (e.g. bolts) need to be traceable to source, and manufacturer certified to the standard specified. Worth checking with the BMAA if you intend to source parts from anyone other than an aviation parts supplier.