Why not use "v=spfv1 a:lotas-smartman.net ?ip4:81.98.88.0/21 -all"?
This would make any mail from your IP an SPF pass, any mail from NTL's Dublin
pool "neutral" (same as having no SPF record) and any mail from elsewhere a
"fail". The IP range should only come into play in the 5 minutes or so after
your IP addresses changes, as that is the TTL of the DNS A record for
lotas-smartman.net.
The "neutral" result is not a long-term solution. Sure it works fine now (when
no one is rejecting neutrals), but (in the future) everyone will set their MTAs
to reject everything except "pass".
Michael R. Brumm