If they do get replacements, maybe they'll send back 109 and 174/5 to their respective depots!
They don't just have 3 extra Solo SRs and a small E200MMC (which will no doubt not get off the sprint & campus shuttle!) remember, with them having 2 spare longer MMCs (107/8), a spare Volvo and Citaro 906 (probably not for much longer but can be forgotten!). As for the DRTs, rarely more than one is off at a time (unless of a serious case of flooding) which shouldn't be too big of an issue! Also correction, 151 is off, not 150.
As just mentioned they do have more than 4 spares, they have 8 as far as I am concerned. Skylink will probably end up either getting those ex-micks or new buses at this rate, with how many miles they're clocking up!