Slightly to my surprise, I have been able to reconfigure the Huawei 556 and "poisoned" it sufficiently to regain admin privileges in such a way that Vodafone cannot get back into the router, by which I mean that it does not hard- reset itself and restore Vodafone's preferred settings.
There is one big drawback though. I have done something, obviously by mistake, which has disabled telephony. Thus if I call the fixed line to which the broadband is attached, I get an automated message saying that the number I am calling is not available; if I try to make an outgoing call from the fixed line, there is no dial tone and then there is an automated message telling me to plug the USB modem into the router which provides an alternative through HSPA.
I essentially followed the instructions given in the Wikipedia Method 2 as described here:
Apart from going to Management/Access Control and deactivating all the boxes under WAN, I also
deactivated the SNMP and TR-069 boxes under LAN
went to Management/SNMP Agent/Configuration and disabled SNMP agent
went to Management/TR-069 agent and disabled it
Then I reconnected the router to the ADSL line and it rebooted perfectly.
Any ideas on
what I need to do to recuperate telephony via the ADSL line?
whether all the steps I took above were really necessary or whether I might tick or enable something that would make it work better?
Many thanks to all again for all your help and any solution to regaining telephony.