Legal research urgently required.

I need some legal research on the accepted date of service of a claim form when it is served by Royal Mail Special Delivery (guaranteed next day delivery). Obviously the document is signed for and hence proof of service is obtained, but is this service by post, DX etc. or personal service under CPR 7.5 (1). I will be looking for relevant case law (if any) on service of documents by Royal Mail Special Delivery (especially claim forms) as well as service by Registered Post and Recorded Delivery. Also how the Interpretation Act 1978 Section 7 may be used in relation to this matter.

Ideally I am looking for a robust argument to support an assertion that Special Delivery can be taken as service was made at the time of signing for the documents.