Welshdresser:
Can't really answer your exact question. Two years ago was the right time to fight this. Really depends whether there is a clause in the loan terms & conditions that requires it. Obtain the agreement and go through it with a fine tooth comb. If you cannot see the applicable clause you will need to ask HSBC to point it out. If they can't I would be very surprised if they are not sympathetic. Complain formally if you are not satisfied.
Next issue, its not obvious that in the circumstances it was right for HSBC to levy a first charge in 6 months and also not to give notice. You need to obtain the charges leaflet, link below but seems a bit long so no idea if it will work.
http://www.hsbc.co.uk/1/PA_1_5_S5/content/uk/pdfs/en/90030_9_bankacctcards_trav_q4_web.pdf;jsessionid=0000qtQpBG7Yk2j9-PbgD2Fc8WC:12c58nvti
Go through the leaflet with another fine tooth comb. If you cannot see the grounds for the charge ask HSBC to explain. Complain formally if you are not satisfied with the explanation.
Good luck.