If not all of your order is actually in stock, what happens next depends upon circumstances. If 80% or more of your order is available for immediate delivery, and the remainder can be delivered by letter post, then we may choose, without reference to you, to deliver your partial order at once. Your invoice for this delivery will state that a further delivery will follow.
The remaining part of your order will be delivered when it becomes available, free of delivery charges.
All other cases of partial availability will be dealt with by arrangement with yourself. Also if you place a further order to be delivered at the same time as your original back-ordered parts, then that will be subject to a further delivery charge since it probably can no longer be delivered by letter post.