Hi Simon, Simon here, I had the same problem earlier on this year, sadly you need to drop the engine out to get the collector box out. If you spray all nuts with WD40 first, (to help you loosen those siezed threads), get a trolley jack under the engine (on a block of wood) and take the weight of the engine then undo and remove all of the engine mounting bolts, take the chain and other conections off then lower the engine and roll it forward. you will then be able to get the collector box off. Its not as bad as it sounds as long as the nuts and bolts undo. Good luck.