A regulation 10 PCN is one that is sent by post and in the case of the Hemel Bus Lane are issued form information obtained by an Approved Device.
What is below is a direct quote from The Civil Enforcement of Parking Contraventions (England) General Regulations 2007 2007 No. 3483 PART 2. See link below
http://www.legislation.gov.uk/uksi/2007/3483/part/2/made--------------------------------------------------------
(4) Subject to paragraph (6), a regulation 10 penalty charge notice may not be served later than the expiration of the period of 28 days beginning with the date on which, according to a record produced by an approved device, or information given by a civil enforcement officer, the contravention to which the penalty charge notice relates occurred (in these Regulations called “the 28-day period”).(5) Paragraph (6) applies where—
(a)within 14 days of the appropriate date the enforcement authority has requested the Secretary of State to supply the relevant particulars in respect of the vehicle involved in the contravention and those particulars have not been supplied before the expiration of the 28-day period; .
(b)an earlier regulation 10 penalty charge notice relating to the same contravention has been cancelled under regulation 23(5)(c); or .
(c)an earlier regulation 10 penalty charge notice relating to the same contravention has been cancelled under regulation 5 of the Representations and Appeals Regulations.
(6) Where this paragraph applies, notwithstanding the expiration of the 28-day period, an enforcement authority shall continue to be entitled to serve a regulation 10 penalty charge notice—
(a)in a case falling within paragraph (5)(a), for a period of six months beginning with the appropriate date; or .
(b)in a case falling within paragraph (5)(b) or (c), for a period of 4 weeks beginning with the appropriate date. .
(7) In this regulation—
(a)“the appropriate date” means— .
(i)in a case falling within paragraph (5)(a), the date referred to in paragraph (4); .
(ii)in a case falling within paragraph (5)(b), the date on which the district judge serves notice in accordance with regulation 23(5)(d); or .
(iii)in a case falling within paragraph (5)(c) the date on which the previous regulation 10 penalty charge notice was cancelled; and .
(b)“relevant particulars” means particulars relating to the identity of the keeper of the vehicle contained in the register of mechanically propelled vehicles maintained by the Secretary of State under the Vehicle Excise and Registration Act 1994. .
Removal of or interference with a penalty charge notice.
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So in certain, very special circumstances they have up to 6 months to serve the NTO. The extra time they allow usually only applies to vehicles not registered to the driver. These may be company cars or hire vehicles etc.
If you are the owner of the car and easily traceable however, my reading of this is that they have 28 days to serve you, or it is out of time and therefore a "procedural impropriety".
