For the benefit of anyone wanting to replace the battery for a Securit 700L and since I couldn't find an answer anywhere else (including the user guide and engineer guide) I will detail here. As a video guide, it is pretty much exactly as explained in this video (for a different system): https://youtu.be/4vsC9jgkVbo
I first tried entering the engineer code, but this triggered the internal alarm, suggesting that the engineer code had been changed from its default value in my system.
Solution: I went with option 1 above.
There are two components to the alarm, the KEYPAD (by the front door) and MAIN PANEL (which is a white metal box about 20 cm square, and should be hidden in an internal cupboard/garage). With the alarm deactivated (not set), remove the front cover from the MAIN PANEL by removing the two screws. The internal alarm sounds when the cover is removed as a response to the tamper being tripped. The alarm is deactivated by resetting the system (enter user code, followed by #) on the KEYPAD. The tamper light will remain lit on the KEYPAD until the cover is replaced on the MAIN PANEL. The earth cable for the MAIN PANEL may be attached to the front cover.
Now replace the battery, by removing the red then the black wires from the terminals and replace with a like battery. In my system it was a 12V, 2.1 Ah lead acid battery, like this: http://amzn.eu/d/5kVETi2. Recommed buying from Screwfix as you can easily return/exchange if needed.
Replace the front cover (tamper light should go out) and reset the alarm just for the hell of it.
Throughout this process I left the mains power on so the external alarm bell (which is powered by a separate backup battery) didn't ring. Best to wear insulating gloves.
Other notes: this is an easy job which costs £12 for a new battery and saves an engineer call out (£200+). Why there aren't instructions for this in the user or engineer guides (bearing in mind it needs doing every 5-10 years) can only be because it is classed as unsafe for a customer to attempt, and too easy to bother mentioning for an engineer. I think Securitech, the original manufacturer, was bought by ADT Security - I did phone them for advice but despite being as helpful as possible they couldn't offer any over the phone assistance.