dbln.cable.ntl.com doesn't have an SPF record, so I think you might be confused
as to how the "include" mechanism works.
Is "ntl.lotas-smartman.net" automatically updated via DDNS whenever your IP
address changes? If so, your TTL is 20 minutes, so potentially your mail could
fail SPF evaluations on some servers for 20 minutes after your IP address
change.
If you can't do DDNS, you could just use the "ip4" mechanism to include the
address space your dynamic IP is allocated from.
For example, if your IP address is always allocated from:
IP: 81.98.88.1-254
netmask: 255.255.255.0
You could do this:
"v=spf1 ip4:81.98.88.0/24 -all"
If your IP address can come from multiple ranges, just include each. You could
call your ISP to find these out. Obviously, this allows anyone in those subnets
to forge your domain, but it's better than nothing. Ideally you want a static
IP or at least relay through a mail server with a static IP.
Michael R. Brumm